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1 Design and Construction of Large, Welded, Low-Pressure Storage Tanks API STANDARD 620 TENTH EDITION, FEBRUARY 2002 ADDENDUM 1, JUNE 20

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3 Design and Construction of Large, Welded, Low-Pressure Storage Tanks Downstream Segment API STANDARD 620 TENTH EDITION, FEBRUARY 2002 ADDENDUM 1, JUNE 20

4 SPECIAL NOTES API publications necessarily address problems of a general nature. With respect to particular circumstances, local, state, and federal laws and regulations should be reviewed. API is not undertaking to meet the duties of employers, manufacturers, or suppliers to warn and properly train and equip their employees, and others exposed, concerning health and safety risks and precautions, nor undertaking their obligations under local, state, or federal laws. Information concerning safety and health risks and proper precautions with respect to particular materials and conditions should be obtained from the employer, the manufacturer or supplier of that material, or the material safety data sheet. Nothing contained in any API publication is to be construed as granting any right, by implication or otherwise, for the manufacture, sale, or use of any method, apparatus, or product covered by letters patent. Neither should anything contained in the publication be construed as insuring anyone against liability for infringement of letters patent. Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least every five years. Sometimes a one-time extension of up to two years will be added to this review cycle. This publication will no longer be in effect five years after its publication date as an operative API standard or, where an extension has been granted, upon republication. Status of the publication can be ascertained from the API Standards department telephone (202) A catalog of API publications, programs and services is published annually and updated biannually by API, and available through Global Engineering Documents, 15 Inverness Way East, M/S C303B, Englewood, CO This document was produced under API standardization procedures that ensure appropriate notification and participation in the developmental process and is designated as an API standard. Questions concerning the interpretation of the content of this standard or comments and questions concerning the procedures under which this standard was developed should be directed in writing to the Director of the Standards department, American Petroleum Institute, 1220 L Street, N.W., Washington, D.C Requests for permission to reproduce or translate all or any part of the material published herein should be addressed to the Director, Business Services. API standards are published to facilitate the broad availability of proven, sound engineering and operating practices. These standards are not intended to obviate the need for applying sound engineering judgment regarding when and where these standards should be utilized. The formulation and publication of API standards is not intended in any way to inhibit anyone from using any other practices. Any manufacturer marking equipment or materials in conformance with the marking requirements of an API standard is solely responsible for complying with all the applicable requirements of that standard. API does not represent, warrant, or guarantee that such products do in fact conform to the applicable API standard. All rights reserved. No part of this work may be reproduced, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission from the publisher. Contact the Publisher, API Publishing Services, 1220 L Street, N.W., Washington, D.C Copyright 20 American Petroleum Institute

5 NOTICE INSTRUCTIONS FOR SUBMITTING A PROPOSED REVISION TO THIS STANDARD UNDER CONTINUOUS MAINTENANCE This standard is maintained under continuous maintenance procedures by the American Petroleum Institute for which the Standards Department. These procedures establish a documented program for regular publication of addenda or revisions, including timely and documented consensus action on requests for revisions to any part of the standard. Proposed revisions shall be submitted to the Director, Standards Department, American Petroleum Institute, 1220 L Street, NW, Washington, D.C , standards@api.org. iii

6 FOREWORD This standard is based on the accumulated knowledge and experience of purchasers and manufacturers of welded, low-pressure oil storage tanks of various sizes and capacities for internal pressures not more than 15 pounds per square inch gauge. The object of this publication is to provide a purchase specification to facilitate the manufacture and procurement of such storage tanks. If tanks are purchased in accordance with the specifications of this standard, the purchaser is required to specify certain basic requirements. The purchaser may desire to modify, delete, or amplify sections of this standard, but reference shall not be made to this standard on the nameplate or manufacturer s certification for tanks that do not fulfill the minimum requirements or that exceed the limitations of this standard. It is strongly recommended that such modifications, deletions, or amplifications be made by supplementing this standard, rather than by rewriting or incorporating sections of it into another complete standard. Each edition, revision, or addenda to this API standard may be used beginning with the date of issuance shown on the cover page for that edition, revision, or addenda. Each edition, revision, or addenda to this API standard becomes effective six months after the date of issuance for equipment that is certified by the manufacturer as being designed, fabricated, constructed, examined, and tested per this standard. During the six-month time between the date of issuance of the edition, revision, or addenda and the effective date, the purchaser and manufacturer shall specify to which edition, revision, or addenda the equipment is to be built. The design rules given in this standard are minimum requirements. More stringent design rules specified by the purchaser or furnished by the manufacturer are acceptable when mutually agreed upon by the purchaser and the manufacturer. This standard is not to be interpreted as approving, recommending, or endorsing any specific design, nor as limiting the method of design or construction. This standard is not intended to cover storage tanks that are to be erected in areas subject to regulations more stringent than the specifications of this standard. When this standard is specified for such tanks, it should be followed insofar as it does not conflict with local requirements. After revisions to this standard have been issued, they may be applied to tanks to be completed after the date of issue. The tank nameplate shall state the date of the edition and any revision to that edition to which the tank is designed and constructed. API publications may be used by anyone desiring to do so. Every effort has been made by the Institute to assure the accuracy and reliability of the data contained in them; however, the Institute makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or damage resulting from its use or for the violation of any federal, state, or municipal regulation with which this publication may conflict. Suggested revisions are invited and should be submitted to the standardization manager, American Petroleum Institute, 1220 L Street, N.W., Washington, D.C iv

7 CONTENTS Page 1 SCOPE General Coverage Limitations REFERENCES DEFINITIONS Stress and Pressure Terms Capacity Terms Tank Wall Welding Terms MATERIALS General Plates Pipe, Flanges, Forging, and Castings Bolting Material Structural Shapes DESIGN General Operating Temperature Pressures Used In Design Loadings Maximum Allowable Stress for Walls Maximum Allowable Stress Values for Structural Members and Bolts Corrosion Allowance Linings Procedure for Designing Tank Walls Design of Sidewalls, Roofs, and Bottoms Special Considerations Applicable to Bottoms That Rest Directly on Foundations Design Of Roof And Bottom Knuckle Regions and Compression-ring Girders Design Of Internal And External Structural Members Shapes, Locations, and Maximum Sizes of Wall Openings Inspection Openings Reinforcement of Single Openings Reinforcement of Multiple Openings Design of Large, Centrally Located, Circular Openings in Roofs and Bottoms Nozzle Necks and Their Attachments to the Tank Bolted Flanged Connections Cover Plates Permitted Types of Joints Welded Joint Efficiency Plug Welds and Slot Welds Stress Relieving Radiographic/Ultrasonic Examination Flush Type Shell Connection FABRICATION General Workmanship Cutting Plates Forming Sidewall Sections and Roof and Bottom Plates v

8 6.5 Dimensional Tolerances Details of Welding Qualification of Welding Procedure Qualification of Welders Matching Plates Cleaning Surfaces to be Welded Weather Conditions for Welding Reinforcement on Welds Merging Weld With Plate Surface Aligning of Main Joints Repairing Defects in Welds Matching Plates of Unequal Thickness Fitting Up of Closure Plates Thermal Stress Relief Peening Field Welds INSPECTION, EXAMINATION AND TESTING Responsibility of Examiner Qualifications of Examiners Access for Inspector Facilities for Inspector Approval of Repairs Inspection of Materials Stamping of Plates Measuring Thickness of Material Inspection of Surfaces Exposed During Fabrication Surface Inspection of Component Parts Check of Dimensions of Component Parts Check of Chemical and Physical Property Data Data Required From Manufacturer on Completed Tanks Check of Stress-Relieving Operation Examination Method and Acceptance Criteria Inspection of Weld Radiographic/Ultrasonic Examination Requirements Standard Hydrostatic and Pneumatic Tests Proof Tests for Establishing Allowable Working Pressures Test Gauges MARKING Nameplates Division of Responsibility Manufacturer s Report and Certificate Multiple Assemblies Page 02 9 PRESSURE- AND VACUUM-RELIEVING DEVICES Scope Pressure Limits Construction of Devices Means of Venting Liquid Relief Valves Marking Pressure Setting of Safety Devices APPENDIX A TECHNICAL INQUIRY RESPONSES A-1 APPENDIX B USE OF MATERIALS THAT ARE NOT IDENTIFIED WITH LISTED SPECIFICATIONS B-1 APPENDIX C SUGGESTED PRACTICE REGARDING FOUNDATIONS C-1 vi

9 Page APPENDIX D SUGGESTED PRACTICE REGARDING SUPPORTING STRUCTURES.... D-1 APPENDIX E SUGGESTED PRACTICE REGARDING ATTACHED STRUCTURES (INTERNAL AND EXTERNAL) E-1 APPENDIX F EXAMPLES ILLUSTRATING APPLICATION OF RULES TO VARIOUS DESIGN PROBLEMS F-1 APPENDIX G CONSIDERATIONS REGARDING CORROSION ALLOWANCE AND HYDROGEN INDUCED CRACKING G-1 APPENDIX H RECOMMENDED PRACTICE FOR USE OF PREHEAT, POST HEAT, AND STRESS RELIEF H-1 APPENDIX I SUGGESTED PRACTICE FOR PEENING I-1 APPENDIX J (RESERVED FOR FUTURE USE) J-1 APPENDIX K SUGGESTED PRACTICE FOR DETERMINING THE RELIEVING CAPACITY REQUIRED K-1 APPENDIX L SEISMIC DESIGN OF STORAGE TANKS L-1 APPENDIX M RECOMMENDED SCOPE OF THE MANUFACTURER'S REPORT M-1 APPENDIX N INSTALLATION OF PRESSURE RELIEVING DEVICES N-1 APPENDIX O SUGGESTED PRACTICE REGARDING INSTALLATION OF LOW PRESSURE STORAGE TANKS O-1 APPENDIX P NDE AND TESTING REQUIREMENTS SUMMARY P-1 APPENDIX Q LOW-PRESSURE STORAGE TANKS FOR LIQUEFIED HYDROCARBON GASES Q-1 APPENDIX R LOW PRESSURE STORAGE TANKS FOR REFRIGERATED PRODUCTS R-1 APPENDIX S AUSTENITIC STAINLESS STEEL STORAGE TANKS S-1 APPENDIX U ULTRASONIC EXAMINATION IN LIEU OF RADIOGRAPHY U-1 Figures 4-1 Isothermal Lines Showing 1-Day Mean Ambient Temperature Minimum Permissible Design Metal Temperature for Pipe, Flanges, and Forgings without Impact Testing Governing Thickness for Impact Test Determination of Pipe, Flanges, and Forgings Biaxial Stress Chart for Combined Tension and Compression, 30,000 38,000 lbf/in. 2 Yield Strength Steels Method for Preparing Lap-Welded Bottom Plates Under the Tank Sidewall Detail of Double Fillet-Groove Weld for Bottom Plates with a Nominal Thickness Greater than 1 / 2 in Typical Free-Body Diagrams for Certain Shapes of Tanks Compression-Ring Region Permissible and Nonpermissible Details of Construction for a Compression-Ring Juncture Reinforcement of Single Openings Part 1 Acceptable Types of Welded Nozzles and Other Connections Part 2 Acceptable Types of Welded Nozzles and Other Connections Part 3 Acceptable Types of Welded Nozzles and Other Connections Part 4 Acceptable Types of Welded Nozzles and Other Connections Large Head Openings and Conical Shell-Reducer Sections Acceptable Types of Flat Heads and Covers Spherically Dished Steel Plate Covers with Bolting Flanges Part 1 Flush-Type Sidewall Connection Part 2 Flush-Type Sidewall Connection Design Factors for Flush-Type Connections Rotation of Sidewall Connection Butt Welding of Plates of Unequal Thickness Nameplate F-1 Reduction of Design Stresses Required to Allow for Biaxial Stress of the Opposite Sign... F vii

10 Page F-2 Examples Illustrating the Use of a Biaxial Stress Chart for Combined Tension and Compression, 30,000 38,000 Pounds per Square Inch Yield Strength Steels F-5 F-3 Form for Use in Graphical Solutions of Problems Involving Biaxial Tension and Compression, 30,000 38,000 Pounds per Square Inch Yield Strength Steels F-6 F-4 Free-Body Sketch F-8 F-5 Example of a Reinforced Opening F-13 F-6 Example of a Reinforced Opening F-15 F-7 Example of a Reinforced Opening F-18 F-8 Example of a Reinforced Opening F-19 L-1 Seismic Zone Map L-3 L-1 Part 2 Seismic Zone Map L-4 L-2 Curves for Obtaining Factors W 1 /W T and W 2 /W T for the Ratio D/H L-5 L-3 Curves for Obtaining Factors X 1 /H and X 2 /H for the Ratio D/H L-5 L-4 Curve for Obtaining Factor k for the Ratio D/H L-5 L-5 Curve for Obtaining the Value of b when M/[D 2 (w t +w L )] Exceeds L-7 Q-1 Typical Stiffening-Ring Weld Details Q-9 Q-2 Radiographic/Ultrasonic Requirements for Butt-Welded Shell Joints in Cylindrical Flat-Bottom Tanks Q-14 R-1 Typical Stiffening-Ring Weld Details R-5 R-2 Radiographic/Ultrasonic Examination Requirements for Butt-Welded Shell Joints in Cylindrical Flat-Bottom Tanks R-11 Tables 4-1 Minimum Requirements for Plate Specifications to be Used for Design Metal Temperatures Maximum Permissible Alloy Content Maximum Allowable Stress Values for Simple Tension Maximum Allowable Efficiencies for Arc-Welded Joints Maximum Allowable Stress Values for Structural Members Sidewall-to-Bottom Fillet Weld for Flat-Bottom Cylindrical Tanks Factors for Determining Values of R 1 and R 2 for Ellipsoidal Roofs and Bottoms Tank Radius Versus Nominal Plate Thickness Allowable Tension Stresses for Uplift Pressure Conditions Minimum Size of Fillet Weld Factors for Determining Values of k for Compression-Ring Bracing Dimensions of Flush-Type Shell Connections (Inches) Sidewall Plate Forming Schedule Diameter Range Versus Radius Tolerance Maximum Thickness of Reinforcement on Welds Stress-Relieving Temperatures and Holding Times Maximum Thickness of Reinforcement on Welds for Radiography Examined Joints F-1 Computed Values of (t c)r, s c, s t, and N for the Assumed Thicknesses: Example F-4 F-2 Computed Values of (t c)r, s c, s t, and N for the Assumed Thicknesses: Example F-7 F-3 Cross-Sectional Area of Standard Angles: Example F-10 L-1 Seismic Zone Tabulation for Some Areas Outside the United States L-1 L-2 Seismic Zone Factor (Horizontal Acceleration) L-2 L-3 Site Coefficients L-6 Q-1 ASTM Materials for Primary Components Q-2 Q-2 Charpy V-Notch Impact Values Q-3 Q-3 Maximum Allowable Stress Values Q-5 Q-4A Minimum Thickness for the Annular Bottom Plate: Steel Tanks Q-7 Q-4B Minimum Thickness for the Annular Bottom Plate: Aluminum Tanks Q-7 Q-5 Nominal Thickness of Inner Tank Cylindrical Sidewall Plates Q-10 Q-6 Radius Tolerances for the Inner Tank Shell Q-10 R-1 Material for Primary Components R-2 R-2 Minimum Charpy V-Notch Impact Requirements for Primary-Component Plate Specimens (Transverse) and Weld Specimens Including the Heat-Affected Zone R-3 R-3 Material for Secondary Components R-3 viii

11 Page R-4 Minimum Permissible Design Metal Temperature for Plates Used as Secondary Components Without Impact Testing R-4 R-5 Minimum Charpy V-Notch Impact Requirements for Secondary-Component Plate Specimens (Transverse) R-7 R-6 Thickness Requirements for the Annular Bottom Plate R-8 S-1a ASTM Materials for Stainless Steel Components (SI Units) S-1 S-1b ASTM Materials for Stainless Steel Components (US Customary Units) S-2 S-2 Maximum Allowable Stress Values for Simple Tension S-3 S-3 Allowable Stresses for Plate Ring Flanges S-4 S-4 Yield Strength Values S-4 S-5 Modulus of Elasticity at the Design Temperature S-4 U-1 Flaw Acceptance Criteria for UT Indications (May be Used for All Materials) U-3 U-2 Alternate Flaw Acceptance Criteria for UT Indications U-4 U-3 Charpy V-notch Impact Values Required to Use Table U-2 for 9% Nickel Steel U-4 ix

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13 Design and Construction of Large, Welded, Low-Pressure Storage Tanks SECTION 1 SCOPE 1.1 GENERAL The API Downstream Segment has prepared this standard to cover large, field-assembled storage tanks of the type described in 1.2 that contain petroleum intermediates (gases or vapors) and finished products, as well as other liquid products commonly handled and stored by the various branches of the industry. The rules presented in this standard cannot cover all details of design and construction because of the variety of tank sizes and shapes that may be constructed. Where complete rules for a specific design are not given, the intent is for the manufacturer subject to the approval of the purchaser s authorized representative to provide design and construction details that are as safe as those which would otherwise be provided by this standard. The manufacturer of a low-pressure storage tank that will bear the API Std 620 nameplate shall ensure that the tank is constructed in accordance with the requirements of this standard. The rules presented in this standard are further intended to ensure that the application of the nameplate shall be subject to the approval of a qualified inspector who has made the checks and inspections that are prescribed for the design, materials, fabrication, and testing of the completed tank. 1.2 COVERAGE This standard covers the design and construction of large, welded, low-pressure carbon steel above ground storage tanks (including flat-bottom tanks) that have a single vertical axis of revolution. This standard does not cover design procedures for tanks that have walls shaped in such a way that the walls cannot be generated in their entirety by the rotation of a suitable contour around a single vertical axis of revolution The tanks described in this standard are designed for metal temperatures not greater than 250 F and with pressures in their gas or vapor spaces not more than 15 lbf/in. 2 gauge The basic rules in this standard provide for installation in areas where the lowest recorded 1-day mean atmospheric temperature is 50 F. Appendix S covers stainless steel low-pressure storage tanks in ambient temperature service in all areas, without limit on low temperatures. Appendix R covers low-pressure storage tanks for refrigerated products at temperatures from + 40 F to 60 F. Appendix Q covers low-pressure storage tanks for liquefied hydrocarbon gases at temperatures not lower than 270 F The rules in this standard are applicable to tanks that are intended to (a) hold or store liquids with gases or vapors above their surface or (b) hold or store gases or vapors alone. These rules do not apply to lift-type gas holders Although the rules in this standard do not cover horizontal tanks, they are not intended to preclude the application of appropriate portions to the design and construction of horizontal tanks designed in accordance with good engineering practice. The details for horizontal tanks not covered by these rules shall be equally as safe as the design and construction details provided for the tank shapes that are expressly covered in this standard Appendix A provides information on the preparation and submission of technical inquiries as well as responses to recent inquiries Appendix B covers the use of plate and pipe materials that are not completely identified with any of the specifications listed in this standard Appendix C provides information on subgrade and foundation loading conditions and foundation construction practices Appendix D provides information about imposed loads and stresses from external supports attached to a tank wall Appendix E provides considerations for the design of internal and external structural supports Appendix F illustrates through examples how the rules in this standard are applied to various design problems Appendix G provides considerations for service conditions that affect the selection of a corrosion allowance; concerns for hydrogen-induced cracking effects are specifically noted Appendix H covers preheat and post-heat stressrelief practices for improved notch toughness Appendix I covers a suggested practice for peening weldments to reduce internal stresses Appendix J is reserved for future use Appendix K provides considerations for determining the capacity of tank venting devices Appendix L covers requirements for the design of storage tanks subject to seismic load.

14 1-2 API STANDARD Appendix M covers the extent of information to be provided in the manufacturer s report and presents a suggested format for a tank certification form Appendix N covers installation practices for pressure- and vacuum-relieving devices Appendix O provides considerations for the safe operation and maintenance of an installed tank, with attention given to marking, access, site drainage, fireproofing, water draw-off piping, and cathodic protection of tank bottoms Appendix P summarizes the requirements for inspection by method of examination and the reference paragraphs within the standard. The acceptance standards, inspector qualifications, and procedure requirements are also provided. This appendix is not intended to be used alone to determine the inspection requirements within this standard. The specific requirements listed within each applicable section shall be followed in all cases Appendix Q covers specific requirements for the materials, design, and fabrication of tanks to be used for the storage of liquefied ethane, ethylene, and methane Appendix R covers specific requirements for the materials, design, and fabrication of tanks to be used for the storage of refrigerated products Appendix S covers requirements for stainless steel tanks in non-refrigerated service Appendix U covers detailed rules for the use of the ultrasonic examination (UT) method for the examination of tank seams. 1.3 LIMITATIONS General The rules presented in this standard apply to vertical, cylindrical oil storage tanks built according to API Standard 650 as specifically allowed in , F.1, and F.7 of that standard. These rules do not apply to tanks built according to rules established for unfired pressure vessels designated for an internal pressure greater than 15 lbf/in. 2 gauge Piping Limitations The rules of this standard are not applicable beyond the following locations in piping connected internally or externally to the walls 1 of tanks constructed according to this standard: a. The face of the first flange in bolted flanged connections. b. The first threaded joint on the pipe outside the tank wall in threaded pipe connections. c. The first circumferential joint in welding-end pipe connections that do not have a flange located near the tank. (All nozzles larger than 2-in. pipe size that are connected to external piping shall extend outside the tank wall a minimum distance of 8 in. and shall terminate in a bolting flange.) 1 The term walls refers to the roof, shell and bottom of a tank as defined in 3.3. Tanks built according to Appendices Q and R may have both an inner and outer roof, shell and bottom. In these doublewall tanks, the piping that (a) may be subjected to the refrigerated product or gas in the annular space between the two tanks and (b) runs through the outer tank to the first circumferential joints must conform to the piping rules stated in Appendices Q and R.

15 SECTION 2 REFERENCES The most recent editions or revisions of the following standards, codes, and specifications are cited in this standard. AA 2 Specifications for Aluminum Structures Allowable Stress Design and Commentary ACI Building Code Requirements for Reinforced Concrete (ANSI/ACI 318) AISC 4 API Spec 5L RP 520 Std 605 Std 650 Std 2000 Manual of Steel Construction Specification for Line Pipe Sizing, Selection, and Installation of Pressure- Relieving Devices in Refineries, Part II, Installation Large-Diameter Carbon Steel Flanges (Nominal Pipe Sizes 26 Through 60; Classes 75, 150, 300, 400, 600, and 900) Welded Steel Tanks for Oil Storage Venting Atmospheric and Low-Pressure Storage Tanks (Non-refrigerated and Refrigerated) ANSI 5 H35.2 Dimensional Tolerances for Aluminum Mill Products ASME 6 B General Purpose (in.) Pipe Threads (ANSI/ ASME B1.20.1) B16.5 Pipe Flanges and Flanged Fittings (ANSI/ ASME B16.5) B31.1 Power Piping B31.3 Chemical Plant and Petroleum Refinery Piping (ANSI/ASME B31.3) B36.10M Welded and Seamless Wrought Steel Pipe (ANSI/ASME B36.10) B96.1 Welded Aluminum-Alloy Storage Tanks (ANSI/ASME B96.1) Boiler and Pressure Vessel Code, Section V, Nondestructive Examination ; Section VIII, Pressure 2 Aluminum Association, th Street, N.W., Washington D.C , 3 American Concrete Institute, P.O. Box Redford Station, Detroit, Michigan 48219, 4 American Institute of Steel Construction, 400 North Michigan Avenue, Chicago, Illinois , 5 American National Standards Institute, 1430 Broadway, New York, New York 10018, 6 American Society of Mechanical Engineers, 345 East 47th Street, New York, New York 10017, ASNT 7 CP-189 Vessels, Division 1 ; and Section IX, Welding and Brazing Qualifications Standard for Qualification and Certification of Nondestructive Testing Personnel SNT-TC-IA Personnel Qualification and Certification in Nondestructive Testing ASTM 8 A 6 General Requirements for Rolled Steel Plates, Shapes, Steel Piling, and Bars for Structural Use A 20 General Requirements for Steel Plates for Pressure Vessels A 27 Steel Castings, Carbon, for General Application A 36 Structural Steel A 53 Pipe, Steel, Black and Hot-Dipped, Zinc- Coated Welded and Seamless A 105 Forging, Carbon Steel, for Piping A 106 A 131 A 134 A 139 A 181 A 182 A 193 A 194 A 213 A 240 A 283 A 285 Components Seamless Carbon Steel Pipe for High-Temperature Service Structural Steel for Ships Pipe, Steel, Electric-Fusion (Arc)-Welded (Sizes NPS 16 and Over) Electric-Fusion (Arc) Welded Steel Pipe ([NPS] in 4 in. and Over) Forgings, Carbon Steel, for General-Purpose Piping Forged or Rolled Alloy-Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service Alloy-Steel and Stainless Bolting Materials for High-Temperature Service Carbon and Alloy Steel Nuts for Bolts for High-Pressure and High-Temperature Service Seamless Ferritic and Austenitic Alloy-Steel Boiler, Superheater, and Heat Exchanger Tubes Heat-Resisting Chromium and Chromium- Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels Low and Intermediate Tensile Strength Carbon Steel Plates Pressure Vessel Plates, Carbon Steel, Lowand Intermediate-Tensile Strength 7 American Society for Nondestructive Testing, 4153 Arlington Plaza, Columbus, Ohio , 8 American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA ,

16 2-2 API STANDARD 620 A 307 Carbon Steel Bolts and Studs, 60,000 psi Tensile Strength A 312 Seamless and Welded Austenitic Stainless Steel Pipe A 320 Alloy Steel Bolting Materials for Low-Temperature Service A 333 Seamless and Welded Steel Pipe for Low-Temperature Service A 334 Seamless and Welded Carbon and Alloy-Steel Tubes for Low-Temperature Service A 350 Forgings, Carbon and Low-Alloy Steel, Requiring Notch Toughness Testing for Piping Components A 351 Castings, Austenitic, Austenitic-Ferritic (Duplex), for Pressure-Containing Parts A 353 Pressure Vessel Plates, Alloy Steel, 9% Nickel, Double-Normalized and Tempered A 358 Electric-Fusion-Welded Austenitic Chromium-Nickel Alloy Steel Pipe for High- Temperature Service A 370 A 403 A 479 A 480 A 516 A 522 A 524 A 537 A 553 A 573 A 633 A 645 A 662 A 671 Test Methods and Definitions for Mechanical Testing of Steel Products Wrought Austenitic Stainless Steel Piping Fittings Stainless Steel Bars and Shapes for Use in Boilers and Other Pressure Vessels General Requirements for Flat-Rolled Stainless and Heat-Resisting Steel Plate, Sheet, and Strip Pressure Vessel Plates, Carbon Steel, for Moderate and Lower Temperature Service Forged or Rolled Eight and 9% Nickel Alloy Steel Flanges, Fittings, Valves and Parts for Low Temperature Service Seamless Carbon Steel Pipe for Atmospheric and Lower Temperatures Pressure Vessel Plates, Heat Treated, Carbon- Manganese-Silicon Steel Pressure Vessel Plates, Alloy Steel, Quenched and Tempered Eight and 9% Nickel Structural Carbon Steel Plates of Improved Toughness Normalized High-Strength Low-Alloy Structural Steel Pressure Vessel Plates, 5% Nickel Alloy Steel, Specially Heat Treated Pressure Vessel Plates, Carbon-Manganese, for Moderate and Lower Temperature Service Electric-Fusion-Welded Steel Pipe for Atmospheric and Lower Temperatures A 673 A 678 A 737 A 841 A 992 B 209 B 210 B 211 B 221 B 241 B 247 B 308 B 444 B 619 B 622 E 23 Sampling Procedure for Impact Testing of Structural Steel Quenched and Tempered Carbon-Steel and High-Strength Low-Alloy Steel Plates for Structural Applications Pressure Vessel Plates, High-Strength, Low- Alloy Steel Steel Plates for Pressure Vessels, Produced by Thermo-Mechanical Process (TMCP) Steel for Structural Shapes for Use in Building Framing Aluminum and Aluminum-Alloy Sheet and Plate Aluminum-Alloy Drawn Seamless Tubes Aluminum and Aluminum-Alloy Bars, Rods, and Wire Aluminum-Alloy Extruded Bars, Rods, Wire, Shapes, and Tubes Aluminum-Alloy Seamless Pipe and Seamless Extruded Tube Aluminum and Aluminum-Alloy Die, Hand and Rolled Ring Forgings Aluminum-Alloy 6061-T6 Standard Structural Shapes, Rolled or Extruded Nickel-Chromium-Molybdenum-Columium Alloys (UNS N06625) Pipe and Tube Welded Nickel and Nickel-Cobalt Alloy Pipe Seamless Nickel and Nickel-Cobalt Alloy Pipe and Tube Notched Bar Impact Testing of Metallic Materials AWS 9 A5.11 Nickel and Nickel Alloy Covered Welding Electrodes (ANSI/AWS A5.11) A5.14 Nickel and Nickel Alloy Bare Welding Rods and Electrodes (ANSI/AWS A5.14) CSA 10 G40.21 Structural Quality Steel ISO Structural Steels 9 American Welding Society, 550 N.W. LeJeune Road, Miami, Florida 33135, 10 Canadian Standards Association, 178 Rexdale Boulevard, Rexdale, Ontario M9W IR3, 11 International Organization for Standardization. ISO publications can be obtained from national standards organizations such as ANSI, 02

17 SECTION 3 DEFINITIONS 3.1 STRESS AND PRESSURE TERMS maximum allowable stress value: The maximum unit stress permitted to be used in the design formulas given or provided for in this standard for the specific kind of material, character of loading, and purpose for a tank member or element (see 5.5 and 5.6) design pressure: The maximum positive gauge pressure permissible at the top of a tank when the tank is in operation. It is the basis for the pressure setting of the safetyrelieving devices on the tank. The design pressure is synonymous with the nominal pressure rating for the tank as referred to in this standard (see 5.3.1). 3.2 CAPACITY TERMS nominal liquid capacity: The total volumetric liquid capacity of a tank (excluding deadwood) between the plane of the high liquid design level and elevation of the tank grade immediately adjacent to the wall of the tank or such other low liquid design level as the manufacturer shall stipulate total liquid capacity: The total volumetric liquid capacity of a tank (excluding deadwood) below the high liquid design level. 3.3 TANK WALL The tank wall is any or all parts of the plates located in the surface of revolution that bounds the tank and serves to separate the interior of the tank from the surrounding atmosphere. Flat bottoms of cylindrical tanks are covered by the rules of As such, the tank walls include the sidewalls (or shell), roof, and bottom of the tank but not any of the following elements located on or projecting from the walls: a. Nozzles and manways or their reinforcement pads or cover plates. b. Internal or external diaphragms, webs, trusses, structural columns, or other framing. c. Those portions of a compression-ring angle, bar, or girder that project from the walls of the tank. d. Miscellaneous appurtenances. 3.4 WELDING TERMS The terms defined in through are commonly used welding terms mentioned in this standard. See 5.22 for descriptions of fusion-welded joints backing: The material metal, weld metal, carbon, granular flux, and so forth that backs up the joint during welding to facilitate obtaining a sound weld at the root base metal: The metal to be welded or cut depth of fusion: The distance that fusion extends into the base metal from the surface melted during welding filler metal: Metal added in making a weld fusion: The melting together of filler metal and base metal, or the melting of base metal only, which results in coalescence heat-affected zone: The portion of the base metal that has not been melted but whose mechanical properties or microstructures have been altered by the heat of welding or cutting joint penetration: The minimum depth a groove weld extends from its face into a joint, exclusive of reinforcement lap joint: A joint between two overlapping members. An overlap is the protrusion of weld metal beyond the bond at the toe of the weld oxygen cutting: A group of cutting processes wherein the severing of metals is effected by means of the chemical reaction of oxygen with the base metal at elevated temperatures. In the case of oxidation-resistant metals, the reaction is facilitated by use of a flux porosity: The existence of gas pockets or voids in metal reinforcement of weld: Weld metal on the face of a groove weld in excess of the metal necessary for the specified weld size slag inclusion: Nonmetallic solid material entrapped in weld metal or between weld metal and base metal undercut: A groove melted into the base metal adjacent to the toe of a weld and left unfilled by weld metal welded joint: A union of two or more members produced by the application of a welding process weld metal: The portion of a weld that has been melted during welding machine welding: Welding with equipment that performs the welding operation under the constant observation and control of a welding operator. The equipment may or may not perform the loading and unloading of the work manual welding: Welding wherein the entire welding operation is performed and controlled by hand semiautomatic arc welding: Arc welding with equipment that controls only the filler metal feed. The advance of the welding is manually controlled welder: One who performs manual or semiautomatic welding welding operator: One who operates machine welding equipment.

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19 SECTION 4 MATERIALS 4.1 GENERAL Material Specifications Materials used in the construction of API Standard 620 tanks shall comply with the specifications in this section (see Appendices Q, R, and S for specific material requirements). Material produced to specifications other than those listed in this section may be used if the material is certified to meet all the requirements of a material specification listed in this section and that its use is approved by the purchaser Materials That Cannot Be Completely Identified Any plate materials or tubular products on hand that cannot be completely identified with a specification listed in this standard, by records satisfactory to the inspector, may be used to construct tanks according to the rules of this standard if the material passes the test prescribed in Appendix B Accessory Pressure Parts All accessory pressure parts, such as pipe fittings, valves, flanges, nozzles, welding necks, welding caps, manhole frames, and covers, shall be made from materials provided for in this standard or in any accepted ANSI standard that covers the particular part. These parts shall be marked with the name or trademark of the manufacturer and any other markings that are required by the applicable standards. Such markings shall be considered the manufacturer s guarantee that the product complies with the material specifications and standards indicated and is suitable for service at the rating indicated. The intent of this paragraph will have been met if, in lieu of the detailed marking on the part itself, the accessory pressure parts have been marked in any permanent or temporary manner that serves to identify the part with the manufacturer s written listing of the particular items and if this listing is available for examination by the inspector Small Parts Cast, forged, or rolled parts of small size (which are ordinarily carried in stock and for which mill test reports or certificates are not customarily furnished) may be used if, in the opinion of the inspector, they are suitable for the purpose intended and that, if such parts are to be welded, they are of welding grade. 4.2 PLATES General All plates that are subject to pressure-imposed membrane stress or are otherwise important to the structural 4-1 integrity of a tank, including bottom plates welded to the cylindrical sidewall of flat-bottom tanks, shall conform to specifications selected to provide a high order of resistance to brittle fracture at the lowest temperature to which the metal in the walls of the tank is expected to fall on the coldest days of record for the locality where the tank is to be installed In all cases, the purchaser shall specify the design metal temperature, and the plates used for the tank shall conform to one or more of the specifications listed in Table 4-1 as being acceptable for use at that temperature. Except as otherwise provided in the last sentence of this paragraph and in 4.2.2, the design metal temperature for materials in contact with nonrefrigerated fluids shall be assumed to be 15 F above the lowest one-day mean ambient temperature for the locality involved, as determined from Figure 4-1. For locations not covered by Figure 4-1, authentic meteorological data shall be used. Where no such data are available, the purchaser shall estimate the temperature from the most reliable information at hand. Where special means, such as covering the outside of the tank with insulation or heating the tank contents, are provided to ensure that the temperature of the tank walls never falls to within 15 F of the lowest one day mean ambient temperature, the design metal temperature may be set at a higher level that can be justified by computations or by actual temperature data on comparable existing tanks Unless exempted per 4.2.2, notch toughness of specially designed plate flanges and cover plates shall be evaluated using governing thickness in Table 4-1. (See for definition of governing thickness Low Stress Design The following design criteria, relative to the use of Table 4-1, apply when the actual stress under design conditions does not exceed one-third of the allowable tensile stress: a. Consideration of the design metal temperature is not required in selecting material from Table 4-1 for tank components that are not in contact with the liquid or vapor being stored and are not designed to contain the contents of an inner tank (see Q.2.3 and R.2.2). b. The design metal temperature may be increased by 30 F in selecting material from Table 4-1 for tank components that are exposed to the vapor from the liquid or vapor being stored and are not designed to contain the contents of an inner tank. c. Excluding bottom plates welded to the cylindrical sidewall of flat-bottom tanks, the plates of a non refrigerated flat-bottom tank, counterbalanced in accordance with , may be constructed of any material selected from Table 4-1.

20 4-2 API STANDARD 620 Eureka Prince Ruppert 0 Clayoquot Victoria San Francisco 5 20 Red Bluff Sacramento Fresno Los Angeles 5 Seattle Portland San Diego Vancouver Reno Kamloops Baker 10 Prince George Penticion Nelson Spokane Boise Las Vegas Phoenix Cranbrook Helena Pocatello Salt Lake City Tuscon 25 Calgary Grand Canyon Edmonton Billings Saskatoon Medicine Hat 5 Havre Lander Santa Fe El Paso Sheridan Prince Albert Williston Cheyenne Denver Pueblo Regina Amarillo The Pas 45 Pierre Bismark North Platte Aberdeen Sioux Falls Wichita Fargo San Antonio Oklahoma City Dallas Winnepeg Minneapolis Sioux City Topeka Joplin Houston 45 Des Moines Keokuk Kansas City Churchill Sioux Lookout International Falls Duluth Springfield Fort Smith St. Louis Little Rock Shreveport Port Arthur Marquette 25 Green Bay 20 Milwaukee Chicago Moline 15 Springfield 10 Jackson Memphis Fort Wayne Nashville New Orleans Kapuskasing Sault St. Marie Chattanooga Birmingham Montgomery Mobile Ludington Detroit 10 Indianapolis Louisville Columbus Knoxville 5 15 Haileybury 10 5 Port Aux Basques Huntsville Cleveland 15 NEWFOUNDLAND 20 Buchans Southhampton London Cincinnati Wythville Atlanta Toronto Charleston Ashville St. Catherine Buffalo Pittsburgh Raleigh Columbia Montreal Ottawa St. Johns Albany Charleston Savannah Jacksonville Tampa Gander Richmond Arvida Quebec 30 Saranac Lake Harrisburg Hartford Baltimore Washington Lennoxville Montpelier 5 10 New York Philadelphia Norfolk Wilmington 15 Chatham Bangor Portland Concord Boston Providence St. John Charlottestown Halifax Sidney Amherst Compiled from U.S. Weather Bureau and Meteorological Div. Dept. of Transport of Dominion of Canada Records up to Miami Figure 4-1 Isothermal Lines Showing 1-Day Mean Ambient Temperature

21 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 4-3 Table 4-1 Minimum Requirements for Plate Specifications to be Used for Design Metal Temperatures Design Metal Temperature (see 4.2.1) Plate Thickness Including Corrosion Allowance (in.) 65 F and over 3 /4 1 > 1 25 F and over 1 /2 1 > 1 5 F and over 1 /2 > 1 /2 35 F and over 1 /2 1 Specification Any listed in ASTM A 36 CSA G40.21 Any listed in ASTM A 36 Mod 2 ASTM A 131 CSA G40.21 Permissible Specifications Grade 38W, 44W, 50W B 38W, 44W, 50W > 1 ASTM A 131 CSA G40.21 ASTM A 131 ASTM A 516 ASTM A 573 ASTM A 662 ASTM A 737 ASTM A 841 CSA G40.21 ISO 630 ASTM A 131 ASTM A 516 ASTM A 537 ASTM A 573 ASTM A 633 ASTM A 662 ASTM A 678 ASTM A 737 ASTM A 841 CSA G40.21 ISO 630 ASTM A 131 ASTM A 516 ASTM A 537 ASTM A 573 ASTM A 633 ASTM A 662 ASTM A 678 ASTM A 737 ASTM A 841 CSA G40.21 ISO 630 ASTM A 131 ASTM A 516 ASTM A 537 ASTM A 573 ASTM A 633 ASTM A 662 ASTM A 678 ASTM A 737 ASTM A 841 CSA G40.21 ISO 630 B 38W, 44W, 50W CS 55, 60, 65, 70 58, 65, 70 B and C B Class 1 38W, 44W, 50W E 275, E355 Quality D CS 55, 60, 65, 70 Classes 1 and 2 58, 65, 70 C and D B and C A and B B Class 1 38W, 44W, 50W E 275, E355 Quality D CS 55, 60, 65, 70 Classes 1 and 2 58 C and D B and C A and B B Class 1 38W, 44W, 50W E275, E355 Quality D CS 55, 60, 65, 70 Classes 1 and 2 58 C and D B and C A and B Class 1 B 38W, 44W, 50W E275, E355 Quality D Special Requirements (in addition to 4.2.3) None None Note 1 None None None None Note 1 None None None Note 1 Note 1 Note 1 None None Note 2 Notes 1 and 2 None None None None None None None None None Note 2 Notes 1 and 2 None Note 3 None Note 3 None Note 3 None None None Notes 2 and 3 Notes 2 and 3 Note 4 Notes 3 and 4 Note 4 Notes 3 and 4 Note 4 Notes 3 and 4 Note 4 Note 4 None Notes 2, 3, and 4 Notes 2, 3, and Notes: 1. All plates over 1 1 /2 in. thick shall be normalized. 2. The steel shall be killed and made with fine-grain practice. 3. The plates shall be normalized or quench tembered (see ). 4. Each plate shall be impact tested in accordance with

22 4-4 API STANDARD Plate Specifications Table 4-2 Maximum Permissible Alloy Content General The specifications listed in through are approved for plates, subject to the modifications and limitations of this paragraph, 4.2.4, and Table ASTM Specifications The following ASTM specifications are approved for plates: a. A 20. b. A 36, with the following API modification as required (see Table 4-1 and Appendix R): Mod 2 requires the manganese content to have a range of The material supplied shall not be rimmed or capped steel. c. A 131 (structural quality only). d. A 283 (Grades C and D only, with a maximum nominal thickness of 3 /4 in.). e. A 285 (Grade C only, with a maximum nominal thickness of 3 /4 in.). f. A 516, with the following API modifications as required (see Appendix R): Mod 1 requires the carbon content to be restricted to a maximum of 0.20% by ladle analysis; a maximum manganese content of 1.50% shall be permitted. Mod 2 requires the minimum manganese content to be lowered to 0.70% and the maximum increased to 1.40% by ladle analysis. The carbon content shall be limited to a maximum of 0.20% by ladle analysis. The steel shall be normalized. The silicon content may be increased to a maximum of 0.50% by ladle analysis. g. A 537, with the following modification: The minimum manganese content shall be 0.80% by ladle analysis. The maximum manganese content may be increased to 1.60% by ladle analysis if maximum carbon content is 0.20% by ladle analysis. h. A 573. i. A 633 (Grades C and D only). j. A 662 (Grades B and C only). k. A 678 (Grades A and B only). l. A 737 (Grade B only). m. A 841 (Class 1 only) CSA Specification The following CSA specification is approved for plates: G40.21 (Grades 38W, 44W, and 50W only; if impact tests are required, these grades are designated 38WT, 44WT, and 50WT). Elements added for grain strengthening shall be restricted in accordance with Table 4-2. Plates shall have a tensile strength not more than 20 ksi above the minimum specified for the grade. Fully killed steel made to a fine grain practice must be specified when required. Alloy % Notes Columbium , 2, and 3 Vanadium , 2, and 4 Columbium (0.05 % maximum) plus vanadium ISO Publication The following ISO publication is approved for plates: 630 (Grades E275 and E355 in Qualities C and D only). For E275, the maximum percentage of manganese shall be 1.50 by ladle analysis. Elements added for grain refining or strengthening shall be restricted in accordance with Table Plate Manufacture , 2, and 3 Nitrogen , 2, and 4 Copper and 2 Nickel and 2 Chromium and 2 Molybdenum and 2 Notes: 1. When not included in the material specification, the use of these alloys, or combinations thereof, shall be at the option of the plate producer, subject to the approval of the purchaser. These elements shall be reported when requested by the purchaser. 2. The material shall conform to these requirements on product analysis subject to the product analyses tolerances of the specification. 3. Columbium, when added either singly or in combination with vanadium, shall be restricted to plates of 0.50-in. maximum thickness unless it is combined with a minimum of 0.15% silicon. 4. When added as a supplement to vanadium, nitrogen (a maximum of 0.015%) shall be reported and the minimum ratio of vanadium to nitrogen shall be 4: All material for plates shall be made using the open-hearth, electric-furnace, or basic-oxygen process. Universal mill plates shall not be used. All plates for pressure parts, with the exception of those whose thicknesses are established by the requirements of Table 5-6, shall be ordered on the basis of edge thickness to ensure that the plates furnished from the mill will not underrun the specified thickness by more than 0.01 in. This stipulation shall not be construed to prohibit the use of plates purchased based on weight if it is established by actual measurements (taken at a multiplicity of points along the edges of the plates) that the minimum thicknesses of the plates do not underrun the required design thickness by more than 0.01 in Subject to the approval of the purchaser, controlled-rolled or thermo-mechanical control process (TMCP) plates (material produced by a mechanical-thermal rolling process designed to enhance the notch toughness) may be used where normalized plates are required. Each plate, as 02

23 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 4-5 rolled, shall be Charpy V-notch tested according to the requirements of R Impact Test Specimens When required by Table 4-1, each plate shall be impact tested; plate refers to the unit plate rolled from a slab or directly from an ingot. The ASTM A 370, Type A, Charpy V- notch test shall be used. The long dimension of the specimen shall be parallel to the direction of the expected maximum stress. When the coincident stresses are approximately equal, the specimens shall be taken transverse to the final direction of the plate rolling. The requirements of R shall be satisfied, except that the minimum energy absorption values of Table R-5 may be substituted for those of Table R PIPE, FLANGES, FORGING, AND CASTINGS All pipe, flanges, forgings, and castings used in the parts of the tanks that are subject to internal pressure shall conform to applicable requirements of to inclusive Pipe Carbon steel pipe shall conform to one of the following specifications: a. ASTM A 53. b. ASTM A 106. c. ASTM A 134, excluding helical (spiral) welded pipe. d. ASTM A 139, excluding helical (spiral) welded pipe. e. ASTM A 333. f. ASTM A 524. g. ASTM A 671 (Grades CA, CC, CD, and CE only). h. API Specification 5L (Grades A and B only) When ASTM A 134, A 139, or A 671 pipe is used, it shall comply with the following: a. The pipe shall be certified to have been pressure tested. b. The plate specification for the pipe shall satisfy the requirements of 4.2.3, 4.2.4, and that are applicable to that plate specification. c. Impact tests for qualifying the welding procedure for the pipe longitudinal welds shall be performed in accordance with Built-Up Fittings Built-up fittings, such as ells, tees, and return bends, may be fabricated by fusion welding when they are designed according to the applicable paragraphs in this standard. 12 For design metal temperatures below 20 F, the materials shall conform to Tables R-1 and/or R Flanges Hub, slip-on welding neck and long welding neck flanges shall conform to the material requirements of ASME B16.5 for forges carbon steel flanges. Plate material used for nozzle flanges shall have physical properties better than or equal to those required by ASME B16.5. Plate flange material shall conform to For nominal pipe sizes greater than 24 in., flanges that conform to ASME B16.47, Series B, may be used, subject to the purchaser s approval. Particular attention should be given to ensuring that mating flanges of appurtenances are compatible Castings and Forgings Large castings and forgings (see Footnote 12 for both materials) not covered in shall be of welding grade if welding is to be done on them, and they shall conform to one of the following ASTM specifications: a. A 27 (Grade 60-30, for structural parts only). b. A 105. c. A 181. d. A Toughness Requirements Except as covered in , the toughness requirements of pipe, flanges, and forgings shall be established as described in through No impact testing is required for ASME B16.5 ferritic steel flanges used at minimum design metal temperature, no colder than 20 F. Piping materials made according to ASTM A 333 and A 350 may be used at a minimum design metal temperatures, no lower than the impact test temperature required by the ASTM specification for the applicable material grade, unless additional impact tests (see ) are conducted Other pipe and forging materials shall be classified under the material groups shown in Figure 4-2 as follows: a. Group I API Spec 5L, Grades A, B, ASTM A 106, Grades A and B; ASTM A 53, Grades A and B; ASTM A 181; and ASTM A 105. b. Group II ASTM A 524, Grades I and II The materials in the groups listed in may be used at nominal thicknesses, including corrosion allowance, at minimum design metal temperatures no lower than those shown in Figure 4-2 without impact testing (see ). The

24 4-6 API STANDARD 620 governing thickness (see Figure 4-3) to be used in Figure 4-2 shall be as follows: a. For butt-welded joints, it is the nominal thickness of the thickest welded joint. b. For corner weld (groove or fillet) or lap welds, it is the thinner of the two parts joined. c. For nonwelded parts (such as bolted flanges), it is 1 4 of flat cover nominal thickness When impact tests are required by or , they shall be performed in accordance with the requirements, including minimum energy requirements of ASTM A 333, Grade 1 for pipe, or ASTM A 350 Grade LF1, for forgings at a test temperature no higher than the minimum design metal temperature. Except for the plate specified in 4.2.3, the material specified in 4.3 shall have a minimum Charpy V-notch impact strength of 13 ft-lbs (full size specimen) at a temperature no higher than the minimum design metal temperature. 4.4 BOLTING MATERIAL Carbon steel bolts 13 may be used if they conform to the following, or to better, 14 specifications: a. ASTM A 193. b. ASTM A 307. c. ASTM A STRUCTURAL SHAPES All structural shapes (see footnote 12) that are subject to pressure-imposed loads or are otherwise important to the structural integrity of a tank shall be made only by the openhearth, electric-furnace, or basic-oxygen process and shall conform to one of the following specifications: a. ASTM A 36 and the following API modification as required (see Appendix R): Mod 1 requires the steel to be made with fine grain practice, with manganese content in the range of % of by ladle analysis. b. ASTM A 131. c. ASTM A 633 (Grade A only). d. ASTM A 992. e. CSA G40.21 (Grades 38W, 44W, and 50W only; if impact tests are required, these grades are designated 38WT, 44WT, and 50WT). 13 For design metal temperatures below 20 F, the materials shall conform to Tables R-1 and/or R If better grades of bolts are used, higher bolt stress values are not recommended with full-faced gaskets. 02

25 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS Design metal temperature ( F) Group I Group II Thickness, including corrosion allowance (inches) Figure 4-2 Minimum Permissible Design Metal Temperature for Pipe, Flanges, and Forgings without Impact Testing

26 ;; ; ;; ; 4-8 API STANDARD 620 T c T c ;;Tf t s T f t n ; ;;; ; t s SLIP ON FLANGE t n T c WELDING NECK FLANGE C L C L 1. Shell reinforcing plate is not included in the illustration above. 2. t s = shell thickness; t n = nozzle neck thickness; T f = flange thickness; T c = bolted cover thickness. 3. The governing thickness for each component shall be as follows: C L C L T c T f Tf t n RING TYPE FLANGE t n t s t s LONG WELDING NECK FLANGE Components Nozzle neck Slip-on flange Ring-type flange Welding-neck flange Long welding-neck flange Bolted cover Governing Thickness (whichever is less) t n or t s t n or T f t n or T f t n or T f t n or t s 1 /4 T c Figure 4-3 Governing Thickness for Impact Test Determination of Pipe, Flanges, and Forgings

27 SECTION 5 DESIGN GENERAL Scope of Rules The rules presented in this standard are intended to establish approved engineering practices for low-pressure storage tanks constructed of any shape within the scope of 1.2 and to provide the fundamental rules for design and testing, which can serve as a sufficient basis for an inspector to judge the safety of any vessel and improve the application of the API 620 nameplate. Where these rules do not cover all details of design and construction, the manufacturer, subject to the approval of the authorized inspector, shall provide details of design and construction that will be as safe as those provided by this standard Pressure Chambers For tanks that consist of two or more independent pressure chambers and have a roof, bottom, or other elements in common, each pressure part shall be designed for the most severe combination of pressure or vacuum that can be experienced under the specified operating conditions Avoidance of Pockets Tank walls shall be shaped to avoid any pockets on the inside where gases may become trapped when the liquid level is being raised or on the outside where rainwater may collect Volume of Vapor Space The volume of the vapor space above the high liquid design level upon which the nominal capacity is based shall be not less than 2% of the total liquid capacity (see 3.2.2) Tests of New Design When a tank is of a new design and has (a) an unusual shape or (b) large branches or openings that may make the stress system around these locations in the tank wall unsymmetrical to a degree that, in the judgment of the designer, does not permit computation with a satisfactory assurance of safety, the tank shall be subjected to a proof test, and straingauge surveys shall be made as provided in OPERATING TEMPERATURE The temperature of the liquids, vapor, or gases stored in, or entering, these tanks shall not exceed 250 F (see 1.2.2) PRESSURES USED IN DESIGN Above Maximum Liquid Level The walls of the gas or vapor space and other tank components that are above the maximum liquid level at the top of the tank shall be designed for a pressure not less than that at which the pressure relief valves are to be set; they shall be designed for the maximum partial vacuum that can be developed in the space when the inflow of air (or another gas or vapor) through the vacuum relief valves is at its maximum specified rate. The maximum positive gauge pressure for which this space is designed shall be understood to be the nominal pressure rating for the tank (sometimes referred to as the design pressure) and shall not exceed 15 lbf/in. 2 gauge When a tank is to operate at liquid levels that at no time reach the top of the roof but the tank will be filled to the top of the roof during the hydrostatic test as provided in , the tank must be designed for both maximum liquidlevel conditions, using in each case the weight of liquid specified in A suitable margin shall be allowed between the pressure that normally exists in the gas or vapor space and the pressure at which the relief valves are set; this margin allows for pressure increases caused by variations in the temperature or gravity of the liquid contents of the tank and by other factors that affect the pressure in the gas or vapor space The maximum partial vacuum will be greater than that at which the vacuum relief valves are set to open Below Maximum Liquid Level All portions of the tank at levels below the aforementioned maximum liquid level shall have each of their important elements designed for at least the most severe combination of gas pressure (or partial vacuum) and static liquid head affecting the element in any specified operation as the pressure in the gas or vapor space varies between the lowest and highest limits encountered during operation Weight for Liquid Storage The weight for liquid storage shall be assumed to be the weight per ft 3 of the specified liquid contents at 60 F, but in no case shall the minimum weight be less than 48 lb/ft 3. This minimum weight does not apply to tanks used for gas storage only, or used for refrigerated liquid storage as discussed in Appendixes Q and R.

28 5-2 API STANDARD LOADINGS The following loadings shall be considered in the design of large, low-pressure storage tanks: a. The internal pressure as specified in 5.3 and any partial vacuum resulting from operation. b. The weight of the tank and specified contents, from empty to full, with or without the maximum gas pressure specified. c. The supporting system, both localized and general, including the effect that is predictable from the nature of the foundation conditions (see Appendices C and D). d. Superimposed loading, such as platforms and brackets for stairways and, where climatic conditions warrant, excessive snow (see Appendix E). e. Wind loads or, when specified, earthquake loadings (see 5.5.6). f. Loads resulting from connected piping. g. The weight of any insulation and linings. 5.5 MAXIMUM ALLOWABLE STRESS FOR WALLS General Higher localized shear and secondary bending stresses may exist in the walls of tanks designed and fabricated according to this standard, and the prescribed test loadings may result in some localized reshaping. This is permissible, since localized reshaping is expected as part of a legitimate fabrication operation, if the reshaping is not so severe that upon release of the test pressure, plastic straining occurs in the opposite direction. This would tend to develop continuing plastic straining in subsequent normal operation Nomenclature Variables relating to stresses common to the requirements of through and Figure 5-1 are defined as follows: t = thickness of the wall, in in., R = radius of the wall, in in., S ts S cs c = corrosion allowance, in in., = maximum allowable stress for simple tension, in lbf/in. 2, as given in Table 5-1, 15 See Biaxial Stress Criteria for Large Low-Pressure Tanks, written by J. J. Dvorak and R.V. McGrath and published as Bulletin No. 69 (June 1961) by the Welding Research Council, 345 East 47th Street, New York, New York = maximum allowable longitudinal compressive stress, in lbf/in. 2, for a cylindrical wall acted upon by an axial load with neither a tensile nor a compressive force acting concurrently in a circumferential direction (determined in accordance with for the thickness-to-radius ratio involved), s ta = allowable tensile stress, in lbf/in. 2 ; s ta lower than S ts because of the presence of a coexistent compressive stress perpendicular to it, s ca s tc s cc s t s c = allowable compressive stress, in lbf/in. 2 ; s ca is lower than S cs because of the presence of a coexistent tensile or compressive stress perpendicular to it, = computed tensile stress, in lbf/in. 2, at the point under consideration, = computed compressive stress, in lbf/in. 2, at the point under consideration, = general variable for indicating a tensile stress, in lbf/in. 2, which may be either an allowable or computed value depending on the context in which the variable is used, = general variable for indicating a compressive stress, in lbf/in. 2, which may be either an allowable or computed value depending on the context in which the variable is used, N = ratio of the tensile stress, s t, to the maximum allowable stress for simple tension, S ts, M = ratio of the compressive stress s c, to the maximum allowable compressive stress, S cs (see Figure F-1) The term tank wall is defined in 3.3. Unless otherwise stipulated in this standard, the stresses in nozzle and manway necks, reinforcing pads, flanges, and cover plates shall not exceed the values that apply for the walls of the tank Maximum Tensile Stresses The maximum tensile stresses in the outside walls of a tank, as determined for any of the loadings listed in 5.4 or any concurrent combination of such loadings that is expected to be encountered in the specified operation, shall not exceed the applicable stress values determined in accordance with provisions described in and If both the meridional and latitudinal unit forces, T 1 and T 2, are tensile or if one force is tensile and the other is zero, the computed tensile stress, S ts, shall not exceed the applicable value given in Table If the meridional force, T 1, is tensile and the coexistent latitudinal unit force, T 2, is compressive or if T 2 is tensile and T 1 is compressive, the computed tensile stress, s tc, shall not exceed a value of the allowable tensile stress, s ta, obtained by multiplying the applicable stress value given in Table 5-1 by the appropriate value of N obtained from Figure 5-1 for the value of compressive stress (s c = s cc ) and the co- 02

29 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 5-3 Figure 5-1 Biaxial Stress Chart for Combined Tension and Compression, 30,000 38,000 lbf/in. 2 Yield Strength Steels related ratio of (t c)/r involved. However, in cases where the unit force acting in compression does not exceed 5% of the coexistent tensile unit force acting perpendicular to it, the designer has the option of permitting a tensile stress of the magnitude specified in instead of complying strictly with the provisions of this paragraph, (see F.1 for examples illustrating the determination of allowable tensile stress values, s ta, in accordance with this paragraph). In no event shall the value of sta exceed the product of the applicable joint efficiency for tension as given in Table 5-2 and the allowable stress for simple tension shown in Table Maximum Compressive Stresses Except as provided in for the compression-ring region, the maximum compressive stresses in the outside walls of a tank, as determined for any of the loadings listed in 5.4 or any concurrent combination of loadings expected to be encountered in the specified operation, shall not exceed the applicable stress values determined in accordance with the provisions described in through These rules do not purport to apply when the circumferential stress on a cylindrical wall is compressive (as in a cylinder acted upon by external pressure). However, values of S cs computed as in , with R equal R 1 when the compressive unit force is latitudinal or to R 2 when the compressive unit force is meridional, in some degree form the basis for the rules given in , , and , which apply to walls of double curvature If a cylindrical wall, or a portion thereof, is acted upon by a longitudinal compressive force with neither a tensile nor a compressive force acting concurrently in a circumferential direction, the computed compressive stress, s cc, shall not exceed a value, S cs, established for the applicable thickness-to-radius ratio as follows: For values of (t c)/r less than , S cs = 1,800,000[(t c)/r] For values of (t c)/r between and , S cs = 10, ,400[(t c)/r] For values of (t c)/r greater than , S cs = 15,000

30 5-4 API STANDARD 620 Table 5-1 Maximum Allowable Stress Values for Simple Tension 02 Specification (See Note 1) Grade Notes ASTM A 36 ASTM A 131 ASTM A 131 ASTM A131 ASTM A 283 ASTM A 283 ASTM A 285 ASTM A 516 ASTM A 516 ASTM A 516 ASTM A 516 ASTM A 537 ASTM A 537 ASTM A 573 ASTM A 573 ASTM A 573 ASTM A 633 ASTM A 662 ASTM A 662 ASTM A 678 ASTM A 678 ASTM A 737 ASTM A 841 CSA G40.21 CSA G40.21 CSA G40.21 CSA G40.21 ISO 630 ISO 630 Seamless API Spec 5L ASTM A 33 ASTM A 106 ASTM A 106 ASTM A 333 ASTM A 333 ASTM A 333 ASTM A 524 ASTM A 524 A B CS C D C Class 1 Class C and D B C A B B Class 1 38W and 38WT 44W and 44WT 50W 50WT E275 Quality C, D E355 Quality C, D B B B C I I1 4 4, 5 and and 5 4, 5 and and and 8 4 and Tensile Strength (lbf/in. 2 ) Plates Pipe 58,000 58,000 58,000 58,000 55,000 60,000 55,000 55,000 60,000 65,000 70,000 70,000 80,000 58,000 65,000 70,000 70,000 65,000 70,000 70,000 80,000 70,000 70,000 60,000 65,000 65,000 70,000 59,500 71,000 60,000 60,000 60,000 70,000 55,000 65,000 60,000 60,000 55,000 Specified Minimum Yield Point (lbf/in. 2 ) 36,000 34,000 34,000 34,000 30,000 33,000 30,000 30,000 32,000 35,000 38,000 50,000 60,000 32,000 35,000 42,000 50,000 40,000 43,000 50,000 60,000 50,000 50,000 38,000 44,000 50,000 50,000 37,000 48,500 35,000 35,000 35,000 40,000 30,000 35,000 35,000 35,000 30,000 Maximum Allowable Tensile Stress for Tension, S ts (lbf/in. 2, see Notes 2 and 3) 16,000 15,200 16,000 16,000 15,200 15,200 16,500 16,500 18,000 19,500 21,000 21,000 24,000 16,000 18,000 19,300 19,300 19,500 21,000 19,300 22,100 21,000 21,000 16,500 18,000 18,000 19,300 16,400 19,600 18,000 18,000 18,000 21,000 16,500 19,500 18,000 18,000 16,500 Electric-Fusion Welded ASTM A 134 ASTM A 134 ASTM A 139 ASTM A 671 ASTM A 671 ASTM A 671 ASTM A 671 ASTM A 671 ASTM A 671 ASTM A 671 ASTM A 671 A 283 Grade C A 285 Grade C B CA55 CC60 CC65 CC70 CD70 CD80 CE55 CE60 4, 5 and 9 5 and and 9 7 and ,000 55,000 60,000 55,000 60,000 65,000 70,000 70,000 80,000 55,000 60,000 30,000 30,000 35,000 30,000 32,000 35,000 38,000 50,000 60,000 30,000 32,000 12,100 13,200 14,400 13,200 14,400 15,600 16,800 16,800 19,200 13,200 14,400

31 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 5-5 Table 5-1 Maximum Allowable Stress Values for Simple Tension (Continued) Specification (See Note 1) Grade Notes Tensile Strength (lbf/in. 2 ) Specified Minimum Yield Point (lbf/in. 2 ) Maximum Allowable Tensile Stress for Tension, S ts (lbf/in. 2, see Notes 2 and 3) ASTM A 105 ASTM A 181 ASTM A 181 ASTM A 350 ASTM A 350 ASTM A 350 ASTM A 27 ASTM A 36 ASTM A 193 ASTM A 307 ASTM A 307 ASTM A 320 ASTM A 36 ASTM A 131 ASTM A 633 ASTM A 992 CSA G40.21 CSA G40.21 CSA G40.21 CSA G40.21 Notes: I II LF1 LF2 LF For anchor bolting B7 B for flanges and pressure parts B for structural parts and anchor bolting L7 A A 38W and 38WT 44W and 44WT 50W 50WT and Forgings 60,000 60,000 70,000 60,000 70,000 70,000 Castings and bolting 60,000 58, ,000 55,000 55, ,000 Structural shapes Resisting Internal Pressure 4 and 6 4 and and 6 4 and 6 4 and 6 4 and 6 4 and 6 58,000 58,000 63,000 65,000 60,000 65,000 65,000 70,000 30,000 30,000 36,000 30,000 36,000 40,000 30,000 36, ,000 18,000 18,000 21,000 18,000 21,000 21, ,000 36,000 34,000 42,000 50,000 38,000 44,000 50,000 50,000 14,400 15,300 24,000 8,400 15,000 24,000 15,200 15,200 17,400 15,200 15,200 15,200 15,200 15, All pertinent modifications and limitations of specifications required by 4.2. through 4.6 shall be complied with. 2. Except for those cases where additional factors or limitations are applied as indicated by references to Notes 4, 6, 10 and 12, the allowable tensile stress values given in this table for materials other than bolting steel are the lesser of (a) 30% of the specified minimum ultimate tensile strength for the material or (b) 60% of the specified minimum yield point. 3. Except when a joint efficiency factor is already reflected in the specified allowable stress value, as indicated by the references to Note 10, or where the value of N determined in accordance with is less than the applicable joint efficiency given in Table 5-2 (and therefore effects a greater reduction in allowable stress than would the pertinent join efficiency factor, if applied), the specified stress values for welds in tension shall be multiplied by the applicable joint efficiency factor, E, given in Table Stress values for structural quality steels include a quality factor of Plates and pipe shall not be used in thickness greater than 3 / 4 in. 6. Stress values are limited to those for steel that has an ultimate tensile strength of only 55,000 lbf/in Less than or equal to 2 1 / 2 in. thickness. 8. Less than or equal to 1 1 / 2 in. thickness. 9. Stress values for fusion-welded pipe include a welded-joint efficiency factor of 0.80 (see ). Only straight-seam pipe shall be used; the use of spiral-seam pipe is prohibited. 10. Stress values for castings include a quality factor of See Allowable stress based on Section VIII of the ASME Boiler and Pressure Vessel Code multiplied by the ratio of the design stress factors in this standard and Section V111 of the ASME Code, namely 0.30/

32 5-6 API STANDARD If both the meridional and latitudinal unit forces, T 1 and T 2, are compressive and of equal magnitude, the computed compressive stress, s cc, shall not exceed a value, s ca, established for the applicable thickness-to-radius ratio as follows: For values of (t c)/r less than , S ca = 1,000,000[(t c)/r] For values of (t c)/r between and , S ca = ,200[(t c)/r] For values of (t c)/r greater than , S ca = If both the meridional and latitudinal unit forces, T 1 and T 2, are compressive but of unequal magnitude, both the larger and smaller computed compressive stresses shall be limited to values that satisfy the following requirements: where (S S s )/S cs S s /S cs 1.0 S l = larger stress, in lbf/in. 2, S s = small stress, in lbf/in. 2, S cs = maximum allowable longitudinal compressive stress, in lbf/in. 2, determined as in using R for the larger unit force in the first equation and for the smaller unit force in the second equation. Note: In the previous expressions, if the unit force involved is latitudinal, R shall be equal to R 1 ; if the force is meridional, R shall be equal to R If the meridional unit force, T 1, is compressive and the coexistent unit force T 2, is tensile, except as otherwise provided in , or if T 2 is compressive and T 1 is tensile the computed compressive stress, s cc, shall not exceed a value of the allowable compressive stress, s ca, determined from Figure 5-1 by entering the computed value of N and the value of t/r associated with the compressive unit stress and reading the value of s c that corresponds to that point. The value of s c will be the limiting value of s ca for the given conditions. (See F-1 for examples illustrating the determination of allowable compressive stress values in accordance with this paragraph.) When a local axial compressive buckling stress in a cylindrical shell is primarily due to a moment in the cylinder, then the allowable longitudinal compressive stress S cs or S ca, as specified in or , may be increased by 20%. If the shell bending is due to wind (tank full or empty) or due to earthquake (tank empty), then in addition to the above allowed 20% increase, the allowable buckling stress due to a moment can be increased an additional 1 / 3. For tanks full or partially full of liquid and for an earthquake induced longitudinal compressive stress, the allowable compression stress need not be limited for biaxial stress as otherwise may be required by Figure 5-1. For seismic design, the tank full is usually the worst case. For wind loading, the tank empty and with internal pressure is usually the worst case for local, bending induced compressive stress The allowable compressive stresses previously specified in are predicated on butt-welded construction. If one or more of the main joints across which the compressive force acts are of the lap-welded type, the allowable compressive stress will be determined according to 5.5.4, but the minimum compressive stress shall be subject to the limitations of and Table 5-2 (including Note 2) Cylindrical shells can be checked for wind buckling to determine if there is the need for intermediate wind girders using the rules of If the transition between the roof or bottom is a curved knuckle section (5.12.3) then 1 / 3 of the knuckle height shall be included as part of the unstiffened shell height Maximum Shearing Stresses The maximum shearing stresses in welds used for attaching manways and nozzles and their reinforcements or other attachments to the walls of a tank and in sections of manway or nozzle necks that serve as reinforcement attachment shall not exceed 80% of the value of the applicable maximum allowable tensile stress, S ts, given in Table 5-1 for the kind of material involved. Such maximum shearing stresses are permissible only where the loading is applied in a direction perpendicular to the length of the weld and must be reduced where the loading is applied differently (see ) Maximum Allowable Stresses for Wind or Earthquake Loadings The maximum allowable stresses for design loadings combined with wind or earthquake loadings shall not exceed 133% of the stress permitted for the design loading condition; except as allowed in Appendix L, this stress shall not exceed 80% of the specified minimum yield strength for carbon steel. For stainless steel and aluminum, see Q

33 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 5-7 Table 5-2 Maximum Allowable Efficiencies for Arc-Welded Joints Type of Joint Butt joints, attained by double-welding or other means approved by the purchaser, that will obtain the quality of deposited weld metal on the inside and outside weld surfaces that agrees with the requirements of Paragraph UW 35 in Section VIII of the ASME Code; welds using metal backing strips that remain in place are excluded. Single-welded butt joint with backing strip or equivalent other than those included above. Single-welded butt joint without backing strip. Double full-fillet lap joint (see Note 4). Single full-fillet lap joint (see Note 4). Single full-fillet lap joints for head-to-nozzle joints Nozzle-attachment fillet welds Plug welds (see ) Limitations None, for all double-welded joints, except for roofs above liquid level. Roofs above liquid level. Longitudinal or meridional circumference or latitudinal joints between plates not more than 1 1 /4 in. thick; nozzle attachment welding without thickness limitation. Roofs above liquid level. Basic Joint Efficiency (%) Radiographed (See Note 1) Spot Full (see Note 3) Spot Full (see Note 3) Spot Full (see Note 3) Spot Full (see Note 3) Maximum Joint Efficiency (%; see Note 2) Nozzle attachment welding Longitudinal or meridional joints and equivalent (see Note 5) circumferential or latitudinal joints between plates not more and 3 /8 in. thick; joints of this type shall not be used for longitudinal or meridional joints that the provisions of require to be butt-welded. Other circumferential or latitudinal joints between plates not more than 5 /8 in thick. Longitudinal or meridional joints and circumferential or latitudinal joints between plates not more than 3 /8 in. thick; joints of this type shall not be used for longitudinal or meridional joints that the provisions of require when the thinner plate joined exceeds 1 /4 in. For attachment of heads convex to pressure not more than 5 /8 in. required thickness, only with use of the fillet weld on the inside of the nozzle. Attachment welding for nozzles and their reinforcements. Attachment welding for nozzle reinforcements (see Note 6) (Included in the strength factors in ) Notes: 1. See 5.26 and 7.15 for examination requirements. 2. Regardless of any values given in this column, the efficiency for lap-welded joints between plates with surfaces of double curvature that have a compressive stress across the joint from a negative value of P g or other external loading may be taken as unity; such compressive stress shall not exceed 700 lbf/in. 2. For all other lap-welded joints, the joint efficiency factor must be applied to the allowable compressive stress, S ca. The efficiency for fullpenetration butt-welded joints, which are in compression across the entire thickness of the connected plates, may be taken as unity. 3. All main butt-welded joints (see ) shall be completely radiographed or ultrasonically examined as specified in 5.26 and nozzle and reinforcement attachment welding shall be examined by the magnetic-particle method as specified in Thickness limitations do not apply to flat bottoms supported uniformly on a foundation. 5. For the purposes of this table, a circumferential or latitudinal joins shall be considered subject to the same requirements and limitations as are longitudinal or meridional joints when such a circumferential or latitudinal joint is located (a) in a spherical, tori spherical or ellipsoidal shape or in any other surface of double curvature, (b) at the junction between a conical or dished roof (or bottom) and cylindrical sidewalls, as considered in or (c) at a similar juncture at either end of a transition section or reducer as shown in Figure The efficiency factors shown for fillet welds and plug welds are not to be applied to the allowable shearing stress values shown in Table 5-3 for structural welds.

34 5-8 API STANDARD MAXIMUM ALLOWABLE STRESS VALUES FOR STRUCTURAL MEMBERS AND BOLTS Subject to the provisions of 5.6.5, the maximum stresses in internal or external diaphragms, webs, trusses, columns, and other framing, as determined for any of the loadings listed in 5.4 or any concurrent combination of such loadings expected to be encountered in the specified operation, shall not exceed the applicable allowable stresses given in Table The slenderness ratio (that is, the ratio of the unbraced length, l, to the least radius of gyration, r) for structural members in compression and for tension members other than rods shall not exceed the following values, except as provided in 5.6.3: a. For main compression members 120. b. For bracing and other secondary members in compression 200. c. For main tension members 240. d. For bracing and other secondary members in tension The slenderness ratio of main compression members inside a tank may exceed 120 but not 200, provided that the member is not ordinarily subject to shock or vibration loads and that the unit stress under full design loadings does not exceed the following fraction of the stress value stipulated in Table 5-3 for the member s actual l/r ratio: f = 1.6 (l/200r) The gross and net sections of structural members shall be determined as described in through The gross section of a member at any point shall be determined by summing the products of the thickness and the gross width of each element as measured normal to the axis of the member. The net section shall be determined by substituting for the gross width the net width which, in the case of a member that has a chain of holes extending across it in any diagonal or zigzag line, shall be computed by deducting from the gross width the sum of the diameters of all holes in the chain and adding the following quantity for each gauge space in the chain: where s g In the case of angles, the gauge for holes in opposite legs shall be the sum of the gauges from the back of the angle minus the thickness In determining the net section across plug or slot welds, the weld metal shall not be considered as adding the net area For splice members, the thickness considered shall be only that part of the thickness of the member that has been developed by the welds or other attachments beyond the section considered In pin-connected tension members other than forged eyebars, the net section across the pinhole, transverse to the axis of the member, shall be not less than 135%; the net section beyond the pinhole, parallel to the axis of the member, shall be not less than 90% of the net section of the body of the member. The net width of a pin-connected member across the pinhole, transverse to the axis of the member, shall not exceed eight times the thickness of the member at the pin unless lateral buckling is prevented External structural, or tubular, columns and framing subject to stresses produced by combination of wind and other applicable loads specified in 5.4 may be proportioned for unit stresses 25% greater than those specified in Table 5-3 if the required section is not less than that required for all other applicable loads combined on the basis of the unit stresses specified in Table 5-3. A corresponding increase may be applied to the allowable unit stresses in the connection bolts or welds for such members Allowable design stresses for bolts are established that recognize possible stressing during initial tightening. For flange bolts, these design allowable stresses also recognize additional stressing during overload and testing. Where bolts are used as anchorage to resist the shell uplift, see for allowable stresses. 5.7 CORROSION ALLOWANCE When corrosion is expected on any part of the tank wall or on any external or internal supporting or bracing members upon which the safety of the completed tank depends, additional metal thickness in excess of that required by the design computations shall be provided, or some satisfactory method of protecting these surfaces from corrosion shall be employed. The added thickness need not be the same for all zones of exposure inside and outside the tank (see Appendix G). s = longitudinal spacing (pitch), in in., of any two successive holes, g = transverse spacing (gauge), in in., of the same two holes. 5.8 LININGS When corrosion-resistant linings are attached to any element of the tank wall, including nozzles, their thickness shall not be included in the computation for the required wall thickness.

35 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 5-9 Table 5-3 Maximum Allowable Stress Values for Structural Members Structural Member Value for Members Not Subject to Pressure-Imposed Loads ( lbf/in. 2 ) Value for Members Subject to Pressure-Imposed Loads ( lbf/in. 2 ) Rolled steel, on net section Butt welds on smaller cross-sectional area, in or at edge of weld (see , item a) Bolts and other threaded parts on net area at roof of thread Axially loaded structural columns, structural bracing, and structural secondary members, on gross section Axially Loaded tubular columns, tubular bracing and tubular secondary members, on gross section (minimum permissible thickness of 1 /4 in.) Butt welds on smaller cross-sectional area, in or at edge of weld (crushing) Plate-girder stiffeners, on gross section Tension on extreme fibres of rolled sections, plate girders, and built-up members Compression on extreme fibers of rolled sections, plate girders, and built-up members With ld/bt not in excess of 600 With ld/bt in excess of 600 Stress on extreme fibers of pins Members subjected to both axial and bending loads shall be proportioned so that maximum combined axial and bending stress will not exceed the permissible value for axial loading alone Tension 18,000 18,000 18,000 Compression (see Note 1) 18,000/[1 + (l 2 /18,000r 2 )] but not to exceed 15,000 18,000Y/[l + (l 2 18,000r 2 )] but not to exceed 15,000Y 18,000 18,000 Bending (see Note 2) 18,000 18,000 10,800,000/(ld/bt) Per Table 5-1 Per Table 5-1 Per Table ,000/[1 + (l 2 /18,000r 2 )] but not to exceed 15,000 18,000Y/[l + (l 2 18,000r 2 )] but not to exceed 15,000Y 15,000 15,000 27,000 Per Table 5-1 Same as tens. val. from Table 5-1 [(600) (tension value from Table 5-1)/[(ld/bt)] 20,000 Stresses on extreme fibers of butt welds resulting from bending shall not exceed the values prescribed for tension and compression, respectively; such values for welds in tension must be multiplied by the applicable joint efficiency Stresses on extreme fibers of butt welds resulting from bending shall not exceed the values prescribed for tension and compression, respectively; such values for welds in tension must be multiplied by the applicable joint efficiency

36 5-10 API STANDARD 620 Table 5-3 Maximum Allowable Stress Values for Structural Members (Continued) Structural Member Value for Members Not Subject to Pressure-Imposed Loads ( lbf/in. 2 ) Shearing (see Note 2) Value for Members Subject to Pressure-Imposed Loads ( lbf/in. 2 ) Pins and turned bolts in reamed or drilled holes Unfinished bolts Webs of beans and plate girders where h/t is not more than 60, or where web is adequately stiffened, on gross section of web Webs of beams and plate finders where web is not adequately stiffened and h/t is more than 60, on gross section of web Fillet welds where load is perpendicular to the length of weld, on the section through the throat (see item b) Fillet welds where load is parallel to the length of weld, on the section through the throat (see , item b) Plug welds or slot welds, on effective faying-surface area of weld (see and Table 5-2) Butt welds on least cross-sectional area, in or at edge of weld (see , item a) Pins and turned bolts in reamed or drilled holes Load applied to bolt at only one side of the member connected Load distributed uniformly, approximately, across thickness of the member connected Unfinished bolts Load applied to bolt at only one side of the member connected Load distributed uniformly, approximately, across thickness of the member connected 13,500 10,000 12,000 18,000/[l + (h 2 /7200t 2 )] 12,600 9,000 11,700 14,400 Bearing 24,400 30,000 16,000 20,000 12,000 8,000 2 / 3 tension value from Table 5-1 (Tension value from Table 5-1) /[l + (h 2 /7200t 2 )] 70% tension value from Table % tension value from Table % tension value from Table % tension value from Table tension value from Table tension value from Table tension value from Table tension value from Table 5-1 Notes: 1. The variables in the compressive stress equations are defined as follows: l = unbraced length of the column, in in.; r = corresponding least radius of gyration of the column, in in.; t = thickness of the tubular column, in in.; Y = unity (1.0) for values of t/r equal to or greater than 0.015; Y = (2/3)[100(t/r)] {2-(2/3)[100)t/r)]} for values of t/r less than The variables in the bending stress equations are defined as follows: l = unsupported length of the member; for a cantilever beam not fully stayed at its outer end against translation or rotation, l shall be taken as twice the length of the compression flange, in in.; d = depth of the member, in in.; b = width of its compression flange, in in.; t = thickness of its compression flange, in in. 3. The variables in the shearing stress equations are defined as follows: h = clear distance between web flanges, in in.; t = thickness of the web, in in. 5.9 PROCEDURE FOR DESIGNING TANK WALLS Free-Body Analysis Free-body analysis denotes a design procedure that determines the magnitude and direction of the forces that must be exerted by the walls of a tank, at the level selected for analysis, to hold in static equilibrium the portion of the tank and its contents above or below the selected level as a free-body, as if it were isolated from the remaining portions of the tank by a horizontal plane cutting the walls of the tank at the level under consideration Levels of Analysis Free-body analyses shall be made at successive levels from the top to the bottom of the tank for the purpose of determining the magnitude and character of the meridional and longitudinal unit forces that will exist in the walls of the tank at critical levels under all the various combinations of gas pressure (or partial vacuum) and liquid head to be encountered in service, which may have a controlling effect on the design. Several analyses may be necessary at a given level of the tank to establish the governing conditions of gas

37 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 5-11 pressure and liquid head for that level. The thicknesses required in the main walls of the tank shall then be computed by the applicable procedures given in Tank Shape and Capacity The analyses in provide the exact shape and overall dimensions needed for the desired capacity of the tank. Except for the more common shapes such as spheres and cylinders, the determination of optimum shapes and sizes is frequently a trial-and-error procedure requiring considerable experience and judgment. As a further preliminary to making the free-body analyses (see 5.9.2) of tanks that will be provided with internal ties, diaphragms, trusses, or other members subject to pressure-imposed loads, studies must be made to establish the preferred arrangement of the members and the magnitude and nature of the loads they must carry under the various conditions of gas pressure and liquid level that will be encountered in operation (see 5.13) Flat Bottoms of Cylindrical Tanks Flat bottoms of cylindrical tanks that are uniformly supported on a ringwall, grade, or concrete-slab foundation are pressure-resisting membranes but are considered nonstressed because of support from the foundation All bottom plates shall have a minimum nominal thickness of 1 /4 in. exclusive of any corrosion allowance specified by the purchaser for the bottom plate. (See Q for an exception to this requirement.) Bottom plates shall be ordered to a sufficient size so that when they are trimmed, at least a 1 in. width will project beyond the outside edge of the weld that attaches the bottom to the sidewall plate Unless otherwise specified by the purchaser, lapwelded bottom plates shall be furnished and installed to lap over the adjacent plate a minimum of 1 in. Three-plate joints in tank bottoms shall not be closer than 12 in. from each other and 12 in. from the sidewall Lap-welded bottom plates under the sidewall shall have the outer ends of the joints fitted and lap-welded to form a smooth bearing for the sidewall plates (see Figure 5-2) Bottom plates under the sidewall that are thicker than 3 /8 in. shall be butt-welded. The butt-welds shall be made using a backing strip 1 /8 in. thick or more, or they shall be butt-welded from both sides. Welds shall be full fusion through the thickness of the bottom plate. The butt-weld shall extend at least 24 in. inside the sidewall Sidewall-to-Bottom Fillet Weld For bottom and annular plate nominal thicknesses 1 /2 in. and less, the attachment between the bottom edge of the lowest course sidewall plate and the bottom plate shall be a continuous fillet weld laid on each side of the sidewall plate. The size of each weld shall not be greater than 1 /2 in., not less than the nominal thickness of the thinner of the two plates joined (that is, the sidewall plate or the bottom plate immediately under the sidewall), and not less than the values shown in Table The plates of the first sidewall course shall be attached to the bottom plates under the sidewall by a fillet weld inside and outside as required by , but when the sidewall material has a specified minimum yield strength greater than 36,000 lbf/in. 2, each weld shall be made with a minimum of two passes For bottom plates under the sidewall with a nominal thickness greater than 1 /2 in., the attachment welds shall be sized so that either the legs of the fillet welds or the groove depth plus the leg of the fillet for a combined weld are of a size equivalent to the thickness of the bottom plate under the sidewall (see Figure 5-3) Discontinuity of Junctures For tanks that have points of marked discontinuity in the direction of the meridional tangent, such as the points that occur at the juncture between a conical or dished roof (or bottom) and a cylindrical sidewall or at the juncture between a conical reducer and a cylindrical sidewall, the portions of the tank near these points shall be designed in accordance with the provisions of DESIGN OF SIDEWALLS, ROOFS, AND BOTTOMS Nomenclature The variables used in the formulas throughout 5.10 are defined as follows: P = total pressure, in lbf/in. 2 gauge, acting at a given level of the tank under a particular condition of loading, = P l + P g, P l = pressure, in lbf/in. 2 gauge, resulting from the liquid head at the level under consideration in the tank, P g = gas pressure, in lbf/in. 2 gauge, above the surface of the liquid. The maximum gas pressure (not exceeding 15 lbf/in. 2 gauge) is the nominal pressure rating of the tank. P g is positive except in computations used to investigate the ability of a tank to withstand a partial vacuum; in such computations, its value is negative, T 1 = meridional unit force, in lbf/in. of latitudinal arc, in the wall of the tank at the level under consideration. T 1 is positive when in tension, T 2 = latitudinal unit force, in lbf/in. of meridional arc, in the wall of the tank at the level under consideration. T 2 is positive when in tension. (In cylindrical sidewalls, the latitudinal unit forces are circumferential unit forces.)

38 5-12 API STANDARD 620 Bottom plate Figure 5-2 Method for Preparing Lap-Welded Bottom Plates Under the Tank Sidewall Figure 5-3 Detail of Double Fillet-Groove Weld for Bottom Plates with a Nominal Thickness Greater than 1 /2 in. (See ) Table 5-4 Sidewall-to-Bottom Fillet Weld for Flat-Bottom Cylindrical Tanks Maximum Thickness of Shell Plate (in.) > > > Shell plate R 1 = radius of curvature of the tank wall, in in., in a meridional plane, at the level under consideration. R 1 is to be considered negative when it is on the side of the tank wall opposite from R 2 except as provided in , R 2 = length, in in., of the normal to the tank wall at the level under consideration, measured from the wall of the tank to its axis of revolution. R 2 is always positive except as provided in , W = total weight, in lb, of that portion of the tank and its contents (either above the level under consideration, as in Figure 5-4, panel b, or below it, as in Figure 5-4, panel a) that is treated as a free-body in the computations for that level. Strictly speaking, the total weight would include the weight of all metal, gas, and liquid in the portion of the tank treated as described; however, the gas weight is negligible, and the metal weight may be negligible compared with the liquid weight. W shall be given the same sign as P when it acts in the same direction as the pressure on the horizontal face of the free-body; it shall be given the opposite sign when it acts in the opposite direction, F = summation, in lb, of the vertical components of the forces in any and all internal or external ties, braces, diaphragms, trusses, columns, skirts, or other structural devices or supports acting on the free-body. F shall be given the same sign as P when it acts in the same direction as the pressure on the horizontal face of the free-body; it shall be given the opposite sign when it acts in the opposite direction, A t = cross-sectional area, in in. 2, of the interior of the tank at the level under consideration, t = thickness, in in., of the sidewalls, roof, or bottom of the tank, including corrosion allowance, c = corrosion allowance, in in., E = efficiency, expressed as a decimal, of the weakest joint across which the stress under consideration acts. [Applicable values given in Table 5-2 shall be used except that, for (a) butt-welded joints in compression across their entire thickness and (b) the lap-welded joints in compression specified in Note 3 of Table 5-2, E may be taken as unity.] S ts = maximum allowable stress for simple tension, in lbf/ in. 2, as given in Table 5-1, s ta = allowable tensile stress, in lbf/in. 2, established as prescribed in , s ca = allowable compressive stress, in lbf/in. 2, established as prescribed in 5.5.4, s tc = computed tensile stress, in lbf/in. 2, at the point under consideration, s cc = computed compressive stress, in lbf/in. 2, at the point under consideration. Minimum Size of Fillet Weld (in.) 3 /16 1 /4 5 /16 3 /8

39 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS Computation of Unit Forces At each level of the tank selected for free-body analysis as specified in 5.9 (see typical diagrams in Figure 5-4) and for each condition of gas and liquid loading that must be investigated at that level, the magnitude of the meridional and latitudinal unit forces in the wall of the tank shall be computed from the following equations, except as provided in , 5.11, or R T 2 1 = ---- P W+ F 2 A t T 2 R 2 P T 1 = ---- R 1 T 2 = R 2 P W+ F 2R 1 2R 1 A t Note: Footnote 16 is also applicable to Equations 1, 2, and 3. R Positive values of T 1 and T 2 indicate tensile forces; negative values indicate compressive forces Free-body analyses shall be made at the level of each horizontal joint in the sidewalls, roof, and bottom of the tank and at any intermediate levels at which the center of curvature changes significantly. The maximum total pressure (liquid head plus gas pressure) that can exist at a given level will not necessarily be the governing condition for that level. Sufficient analyses shall be made at each level to determine the combination of liquid head and gas pressure (or partial vacuum) that, in conjunction with the allowable tensile and compressive stresses, will control the design at that level. A 16 Equations 2, 5, and 9 have been derived from a summation of the normal-to-surface components of the T 1 and T 2 forces acting on a unit area of the tank wall subjected only to pressure P. To be technically correct, the normal-to-surface components of other loads, such as metal, snow or insulation, should be added to or subtracted from P. For the usual internal design pressure, these added loads are small compared with P and can be mooted without significant error. Where the pressure P is relatively small, as in the case of a partial vacuum loading, the other load components can have a substantial effect on the calculated T 2 force and the resultant thickness. Equations 3 and 6 are correct only when P is the free-body pressure without the normal-to-surface components of other loads. The example in F.3 calculates the required roof thicknesses under a small vacuum by considering the metal, insulation and snow loads in Equations 1-5. The designer should note that if these loads had been omitted, the calculated thicknesses would have been much less than the correct values. In Equations 1, 4, 8, and 10, W is intended to include loads of insignificant value, such as metal weight. At points away from the vertical centerline of the roof, the value of T 2 is required for the thickness calculations of Equations 18, 20, and 22 and the value of P in Equations 2, 5, and 9 must be modified by the normal components of the added loads for the correct determination of T 2. R 2 (1) (2) (3) tank may normally be operated at a fixed height of liquid contents, but the tank must be made safe for any conditions that might develop in filling or emptying the tank. This will necessitate a particularly careful investigation of sidewalls of double curvature Mathematically exact instead of approximate values of R 1 and R 2 should be used in computations for ellipsoidal roofs and bottoms. The values for a point at a horizontal distance, x, from the vertical axis of a roof or bottom in which the length of the horizontal semiaxis, a, is two times the length of the vertical semiaxis, b, may be determined by multiplying length a by the appropriate factor selected from Table 5-5. Values for ellipsoidal shapes of other proportions may be computed using the following formulas: b 2 R a a x b 2 ( R ) 3 = = Equations 1 and 2 are general formulas applicable to any tank that has a single vertical axis of revolution and to any free-body in the tank that is isolated by a horizontal plane which intersects the walls of the tank in only one circle (see ). For tanks or segments of tanks of the shapes most commonly used, Equations 1 and 2 reduce to the following simplified equations for the respective shapes indicated in items a-c. a. For a spherical tank or a spherical segment of a tank, R 1 = R 2 = R s (the spherical radius of the tank or segment), and Equations 1 and 2 become the following: a 4 b 2 Note: See Footnote 16 for information applicable to Equations 4-6. b 2 a 4 R 2 = a x 2 T 1 b 2 R s b 2 Furthermore, if the sphere is for gas pressure only and if (W + F)/A t is negligible compared with P g, Equations 4 and 5 reduce to the following: 0.5 = ---- P W+ F 2 A t T 2 = R s P T 1 R T 1 2 = ---- P W+ F 2 A t (4) (5) (6) T 1 = T 2 = 1 /2 P g R s (7) a 4

40 5-14 API STANDARD 620 Axis of revolution Axis of revolution b. For a conical roof or bottom, where T 1 Free body Figure 5-4 Typical Free-Body Diagrams for Certain Shapes of Tanks R 1 = infinity R 2 = R 3 /cos α R 3 = horizontal radius of the base of the cone at the level under consideration, α=οne-half the included apex angle of the conical roof or bottom. For this condition, Equations 1 and 2 reduce to the following: P g W R s Supports W 1 W Panel a 1 Level under consideration T 1 Free body T R W+ F = P cosα R 1 T 1 F/n R 2 Level under consideration A t (8) W Panel c P Ties T 1 P T 1 F/n W 1 Axis of revolution Supports Panel b T 1 R s W 1 n = number of ties cut by this plane Free body c. For cylindrical sidewalls of a vertical tank, R 1 = infinity; R 2 = R c, the radius of the cylinder; and Equations 1 and 2 become the following: T 1 T 2 R c ---- W+ F = P = PR c Note: See Footnote 16 for information applicable to Equation 10. (10) (11) Furthermore, if the cylinder is for gas pressure only and (W + F)/A t is negligible compared with P g, Equations 10 and 11 reduce to the following: A t T 1 = 1 /2 P g R c (12) T 2 = P g R c (13) PR T 3 2 = cosα Note: See Footnote 16 for information applicable to Equations 8 and 9. (9) Where a horizontal plane that passes through a tank intersects the roof or bottom in more than one circle, thus isolating more than one free-body at that level, the formulas given in and apply only to the central free-body whose walls continue across and are pierced by the

41 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 5-15 axis of revolution. (An example of the kind of plane described would be one passed through the bottom of the tank shown in Figure 5-4, panel c, just a short distance below the lower ends of the internal ties.) The meridional and latitudinal unit forces acting along the edges of the annular free-body or bodies lying outside of the central free-body must be computed from formulas developed especially for the particular shape of free-body cross section involved. This standard cannot provide formulas for all shapes of cross sections and conditions of loading that might be used at these locations; however, for a toroidal segment that rests directly on its foundation (see ) and has a constant meridional radius, R 1, such as is used in the outer portion of the bottom of the tanks shown in Figure 5-4, panel c, applicable equations for the meridional and latitudinal unit forces in the walls of the segment are as follows: T 1 = P g R R 2 1 T 2 = --P g R 1 2 (14) (15) The variables are defined in ; however, in this case, R 1 is always positive and R 2 is negative when it is on the tank wall on the side opposite from R Required Thickness The thickness of the tank wall at any given level shall be not less than the largest value of t as determined for the level by the methods prescribed in through In addition, provision shall be made by means of additional metal, where needed, for the loadings other than internal pressure or possible partial vacuum enumerated in 5.4. If the tank walls have points of marked discontinuity in the direction of the meridional tangent, such as occur at the juncture between a conical or dished roof (or bottom) and a cylindrical sidewall, the portions of the tank near these points shall be designed in accordance with the provisions of If the units forces T 1 and T 2 are both positive, indicating tension, for the governing combination of gas pressure (or partial vacuum) and liquid head at a given level of the tank, the larger of the two shall be used for computing the thickness required at that level, as shown in the following equations: T 1 (16) In these equations, S ts and E have the applicable values prescribed in Tables 5-1 and 5-2, respectively. R 1 T 2 t = c or t = c S ts E S ts E Table 5-5 Factors for Determining Values of R 1 and R 2 for Ellipsoidal Roofs and Bottoms (see ) x/a u = R t /a v = R 2 /a Note: The variables in this table are defined as follows: x = horizontal distance from point in roof or bottom to the axis of the revolution; a = horizontal semiaxis of the elliptical cross section; R 1 = ua; R 2 = va If the unit force T 1 is positive, indicating tension, and T 2 is negative, indicating compression, for the governing combination of gas pressure (or partial vacuum) and liquid head at a given level of the tank or if T 2 is positive and T 1 is negative, the thickness of tank wall required for this condition shall be determined by assuming different thicknesses until one is found for which the simultaneous values of the computed tension stress, s tc, and the computed compressive stress, scc, satisfy the requirements of and , respectively. The determination of this thickness will be facilitated by using a graphical solution such as the one illustrated in F Notwithstanding the foregoing provisions, if the unit force acting in compression in the case described does not exceed 5% of the coexistent tensile unit force acting perpendicular to it, the designer has the option of determining the thickness required for this condition by using the method specified in instead of complying strictly with the provisions of this paragraph. The value of the joint efficiency factor, E, will not enter into this determination unless the 17 See Figure F-3, a copy of a chart used to make graphical solutions.

42 5-16 API STANDARD 620 magnitude of the allowable tensile stress, s ta, is governed by the product ES ts as provided in If the unit forces T 1 and T 2 are both negative and of equal magnitude for the governing condition of loading at a given level of the tank, the thickness of tank wall required for this condition shall be computed using Equation 17: T 1 S ca T 2 S ca t = c = c (17) In this equation, S ca has the appropriate value for the thickness-to-radius ratio involved, as prescribed in and Lap-welded joints shall be subject to the limitations of and Table 5-2 (including Note 3) If the unit forces T 1 and T 2 are both negative but of unequal magnitude for the governing condition of loading at a given level, the thickness of tank wall required for this condition shall be the largest of those thickness values, computed by the stepwise procedure outlined in items a f, that show a proper correlation with the respective thickness-to-radius ratios involved in their computation (see Steps 2 and 4). a. Step 1. The values of Equations 18 and 19 shall be computed as follows: t = ( T + 0.8T )R c 1342 Note: See Footnote 16 for information applicable to Equation 18. t = T R c 1000 (18) (19) In both equations, the value of T shall be equal to the larger of the two coexistent unit forces; the value of T shall be equal to the smaller of the two unit forces. R and R shall be equal to R 1 and R 2, respectively, if the larger unit force is latitudinal; conversely, R and R shall be equal to R 2 and R 1, respectively, if the larger unit force is meridional. b. Step 2. The corrosion allowance shall be deducted from each of the two thicknesses computed in Step 1, and the thickness-to-radius ratio, (t c)/r, shall be checked for each thickness based on the value of R used in computing it by either Equation 18 or 19. If both such thickness-to-radius ratios are less than , the larger of the two thicknesses computed in Step 1 will be the required thickness for the condition under consideration; otherwise, Step 3 shall be followed. c. Step 3. If one or both thickness-to-radius ratios determined in Step 2 exceed , the values of the following equations shall be computed: T + 0.8T t = c 15, 000 Note: See Footnote 16 for information applicable to Equation 20. T t = c 8340 (20) (21) d. Step 4. The corrosion allowance shall be deducted from each of the two thicknesses computed in Step 3, and the thickness-to-radius ratio, (t c)/r, shall be checked for each thickness using a value of R equal to R as defined in Step 1 in connection with the thickness determined from Equation 20 and a value of R equal to R connection with the thickness determined from Equation 21. If both such thickness-toradius ratios are greater than , the larger of the two thicknesses computed in Step 3 will be the required thickness for the condition under consideration; otherwise, Step 5 shall be followed. e. Step 5. If one or more of the thickness-to-radius ratios determined in Step 2 or Step 4 fall between and and the thickness involved was computed using Equations 18 or 20, a thickness shall be found that satisfies the following equation: 10, 150( t c) + 277, 400 ( t c) = T + 0.8T R Note: See Footnote 16 for information applicable to Equation 22. (22) If the thickness involved was computed using Equation 19 or 21, a thickness shall be found that satisfies the following equation: 5650( t c) + 154, 200 ( t c) = T R (23) f. Step 6. A tentative final selection of thickness shall be made from among the thickness values computed in the previous steps (if the value has not been finally established earlier in the procedure). The values of s cc shall be computed for both T 1 and T 2 and checked to see that they satisfy the requirements of and If the tentative thickness does not satisfy these requirements, the necessary adjustments shall be made in the thickness to make the values of s cc satisfy these requirements The procedure described in is for the condition in which biaxial compression with unit forces of unequal magnitude is governing. In many cases, however, a tentative thickness will have been previously established by other design conditions and will need to be checked only for

43 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 5-17 the external pressure or partial vacuum condition. In such cases, the designer has only to compute the values of s cc for both T 1 and T 2 and then check to see that these satisfy the requirements of , as specified in Step 6. (See F.3 for examples illustrating the application of ) Intermediate Wind Girders for Cylindrical Sidewalls The maximum height of unstiffened sidewall, in ft, shall not exceed: Least Permissible Thicknesses Tank Wall The minimum thickness of the tank wall at any level shall be the greatest of the following: a. A measure of 3 /16 in. plus the corrosion allowance. b. The calculated thickness in accordance with plus the corrosion allowance. c. The nominal thickness as shown in Table 5-6. The nominal thickness refers to the tank shell as constructed. The thicknesses specified are based on erection requirements Nozzle Neck See for the minimum thickness of the nozzle neck External Pressure Limitations The thicknesses computed using the formulas and procedures specified in 5.10, where P g is a negative value equal to the partial vacuum for which the tank is to be designed, will ensure stability against collapse for tank surfaces of double curvature in which the meridional radius, R 1, is equal to or less than R 2 or does not exceed R 2 by more than a very small amount. Data on the stability of sidewall surfaces of prolate spheroids are lacking; the formulas and procedures are not intended to be used for evaluating the stability of such surfaces or of cylindrical surfaces against external pressure This standard does not contain provisions for the design of cylindrical sidewalls that are subject to partial internal vacuum in tanks constructed for the storage of gases or vapors alone. However, cylindrical sidewalls of vertical tanks designed in accordance with these rules for storing liquids (with the thickness of upper courses not less than specified in for the tank size involved and with increasing thickness from top to bottom as required for the combined gas and liquid loadings) may be safely subjected to a partial vacuum in the gas or vapor space not exceeding 1 ounce per square in. with the operating liquid level in the tank at any stage from full to empty. The vacuum relief valve or valves shall be set to open at a smaller partial vacuum so that the 1-ounce partial vacuum will not be exceeded when the inflow of air (or gas) through the valves is at the maximum specified rate. where H 1 = vertical distance between the intermediate wind girder and the top of the sidewall or in the case of formed heads the vertical distance between the intermediate wind girder and the head-bend line plus one-third the depth of the formed head, in ft, t = the thickness of the top sidewall course, as ordered condition unless otherwise specified, in in., D = nominal tank diameter, in ft. Note: This formula is based on the following factors: a. A design wind velocity, V, of 100 mph which imposes a dynamic pressure of 25.6 lbf/ft 2. The velocity is increased by 10% for either a height above the ground or a gust factor. The pressure is thus increased to 31 lbf/ft 2. An additional 5 lbf/ft 2 is added for internal vacuum. This pressure is intended by these rules to be the result of a 100 miles per hour fastest mile velocity at approximately 30 ft above the ground. H 1 may be modified for other wind velocities, as specified by the purchaser, by multiplying the formula by (100/V) 2. When a design wind pressure, rather than a wind velocity, is stated by the purchaser, the preceding increase factors should be added, unless they are contained within the design wind pressure. b. The formula is based on the wind pressure being uniform over the theoretical buckling mode in the tank sidewall which eliminates the necessity of a shape factor for the wind loading. c. The formula is based on the modified U.S. Model Basin formula for the critical uniform external pressure on thin-wall tubes free from end loading, subject to the total pressure in item a. d. When other factors are specified by the purchaser which are greater than those in (a) through (c), the total load on the sidewall shall be modified accordingly and H 1 shall be decreased by the ratio of 36 lbf/ft 2 to the modified total pressure. e. The background for the criteria given in the note is covered in R. V. McGrath, Stability of API Standard 650 Tank Shells, Proceedings of the American Petroleum Institute, Section III Refining, American Petroleum Institute, New York, 1963, Vol. 43, pp H 1 = 6( 100t) 100t D To determine the maximum height H 1 of the unstiffened sidewall, a calculation shall be made using the

44 5-18 API STANDARD 620 thickness of the top sidewall course. Next the height of the transformed sidewall shall be calculated as follows: a. Change the width (W) of each sidewall course into a transposed width (W tr ) of each sidewall course, having the top sidewall thickness, by the following relationship: where Table 5-6 Tank Radius Versus Nominal Plate Thickness Tank Radius (ft) 25 > > > 100 W tr w t uniform = t actual Nominal Plate Thickness (in.) 3 /16 t uniform = thickness of the top sidewall course, as ordered condition in in.es, unless otherwise specified, t actual = thickness of the sidewall course for which transposed width is being calculated, as ordered condition in in., unless otherwise specified, W = actual course width, in ft, W tr = transposed course width, in ft. b. The sum of the transposed width of each course will give the height of the transformed sidewall If the height of the transposed sidewall is greater than the maximum height, H 1, an intermediate girder is required. a. For equal stability above and below the intermediate wind girder, the latter should be located at the mid-height of the transposed sidewall. The location of the girder on the actual sidewall should be at the same course and relative position as on the transposed sidewall using the foregoing thickness relationship. b. Other locations for the girder may be used provided the height of the unstiffened sidewall on the transposed sidewall does not exceed H 1 (see ). 1 /4 5 /16 3 / If half the height of the transposed sidewall exceeds the maximum height, H 1, a second intermediate girder shall be used in order to reduce the height of unstiffened sidewall to a height less than the maximum Intermediate wind girders shall not be attached to the sidewall within 6 in. of a horizontal joint of the sidewall. When the preliminary location of a girder is within this distance from a horizontal joint, the girder shall preferably be located 6 in. below the joint, except that the maximum unstiffened sidewall height shall not be exceeded The required minimum section modulus, in in. cubed, of the intermediate wind girder shall be determined by the equation: Z = D 2 H 1 Note: This equation is based on wind velocity of 100 miles per hour. If specified by the purchaser, other wind velocities may be used by multiplying the equation by (V/100) 2. Refer to item a. of notes to for a description of the loads on the tank sidewall which are used for the 100 mile per hour design wind velocity Where the use of a transposed sidewall permits the intermediate wind girder to be located at a height less than H 1 calculated by the formula in , the spacing to the mid-height of the transposed sidewall, transposed to the height of the actual sidewall, may be substituted for H 1 in the calculation for minimum section modulus if the girder is attached at the transposed location The section modulus of the intermediate wind girder shall be based upon the properties of the attached members and may include a portion of the sidewall for a distance of 1.47(Dt) 0.5 above and below the attachment to the sidewall, where t is the sidewall thickness at the attachment Intermediate stiffeners extending a maximum of 6 in. from the outside of the sidewall are permitted without need for an opening in the stiffener when the nominal stairway width is at least 24 in. For greater outward extensions of a stiffener, the stairway shall be increased in width to provide a minimum clearance of 18 in. between the outside of the stiffener and the handrail of the stairway, subject to the approval of the purchaser. If an opening is necessary, the built up section shall have a section modulus greater than or equal to that required for the stiffener SPECIAL CONSIDERATIONS APPLICABLE TO BOTTOMS THAT REST DIRECTLY ON FOUNDATIONS Shaped Bottom Where the bottom of a tank is a spherical segment or a spherical segment combined with one or more toroidal segments, or is conical in shape, and the entire bottom area rests directly on the tank foundation in such a way that the foundation will absorb the weight of the tank contents without significant movement, the liquid head may be neglected in computing the internal pressure, P, acting on the bottom and in computing the unit forces, T 1 and T 2, in the bottom. Under these conditions, the unit forces in the bottom of the tank may be computed considering that P in each case is equal to P g.

45 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS Flat-Bottom Tanks with Counterbalance General In tanks that have cylindrical sidewalls and flat bottoms, the uplift that results from the pressure acting on the underside of the roof combined with the effect of design wind pressure, or seismic loads if specified, must not exceed the weight of the sidewalls plus the weight of that portion of the roof that is carried by the sidewalls when no uplifts exists unless the excess is counteracted by a counterbalancing structure such as a concrete ringwall, a slab foundation, or another structural system. The means for accomplishing this shall be a matter of agreement between the manufacturer and the purchaser. Similar precautions must be taken with flat-bottomed tanks of other shapes. All weights used in such computations shall be based on net thicknesses of the materials, exclusive of corrosion allowance Counterbalancing Structure The counterbalancing structure, which may be a foundation or support system, shall be designed to resist uplift calculated as described in based on 1.25 times the internal design pressure plus the wind load on the shell and roof based on its projection on a vertical plane. If seismic loads are specified, uplift shall be calculated using internal design pressure plus the seismic loads. Wind and seismic loads need not be combined Anchorage The design of the anchorage and the attachments to the tank shall be a matter of agreement between the manufacturer and the purchaser and shall satisfy the following conditions: a. The design stresses shall satisfy all of the conditions in Table 5-7. b. When corrosion is specified for the anchors, thickness shall be added to the anchors and the attachments. If bolts are used for anchors, the nominal diameter shall be not less than 1 in. plus a corrosion allowance of at least 1 /4 in. on the diameter. c. Attachments of anchors to the shell shall be designed using good engineering practice. d. Anchor materials and allowable stresses shall be those permitted by Table Flat-Bottom Tanks Without Counterbalancing Weight The detailed design of flat-bottom tanks without counterbalancing weight shall be a matter of agreement between the manufacturer and the purchaser and shall satisfy the following conditions: a. The bottom of a flat-bottom tank shall be designed to remain flat during all design and test conditions. When the flat-bottom tank is designed without anchoring the shell to the counterbalancing weight, the bottom will be designed to carry all the weight and pressure forces distributed on the bottom and to transfer the uplift forces from the sidewall through the bottom plates. The uplift forces will be obtained from a freebody analysis as specified in 5.9 and These forces shall be determined for the tank (deducting any specified corrosion allowance) for both a full and an empty condition and shall include uplift from design wind velocity. The largest values will be used for design. b. The bottom plates in the flat-bottom tank shall be designed as a strength member to span between main structural members (for example, grillage beams or other structural members) and transfer the distributed pressure and liquidweight forces to these main structural members. c. When the bottom plate is a bending strength member, single-fillet lap joints are not permitted in the bottom plate. d. Adequate provision shall be made at the sidewall to transfer the uplift forces from the shell to the shear-carrying elements in the bottom structure. e. Consideration shall be given to protecting all bottom structural elements from environmental corrosion. f. Anchorage shall be provided for resistance to wind and earthquake forces and shall be designed in accordance with Table 5-7 Allowable Tension Stresses for Uplift Pressure Conditions (see ) Source of Uplift Pressure Tank design pressure Tank design pressure plus wind or earthquake Tank test pressure Allowable Tension Stress a (lbf/in. 2 ) Allowable design stress, S ts (see Table 5-1) Smaller of 1.33 S ts or 80% of the specified minimum yield strength Smaller of 1.33 S ts or 80% of the specified minimum yield strength Note: The allowable stresses for stainless steel and aluminum anchors for the applicable loading conditions are found in Q.3.3.5, Q.8.1.3, and Table Q-3. Note: a The allowable tension stress determined at the minimum net section or tensile stress area of the anchor.

46 5-20 API STANDARD Additional Considerations Unless otherwise required, tanks that may be subject to sliding due to wind shall use a maximum allowable sliding friction of 0.40 times the force against the tank bottom DESIGN OF ROOF AND BOTTOM KNUCKLE REGIONS AND COMPRESSION-RING GIRDERS Design Limitations The design rules in this section do not cover the junction between a conical reducer and cylindrical sidewalls except as indicated on Figure 5-9, panel b. However, the provisions of this section shall be observed at such a juncture if the angle formed is nonreentrant. (See for design of reentrant junctures.) General When the roof or bottom of a pressure tank is a cone or partial sphere (or nearly so) and is attached to cylindrical sidewalls, the membrane stresses in the roof or bottom pull inward on the periphery of the sidewalls. This pull results in circumferential compressive forces at the juncture, which may be resisted either by a knuckle curvature in the roof or bottom or by a limited zone at the juncture of the intersecting roof or bottom plates and sidewall plates, supplemented in some cases by an angle, a rectangular bar, or a horizontally disposed ring girder. All longitudinal and meridional joints in a knuckle region, or between those portions of plates that are considered to participate 18 in resisting compressive forces in a compression-ring region, and all radial joints in a compression-ring angle, bar, or girder shall be butt-welded Knuckle Regions If a curved knuckle is provided, a ring girder or other form of compression ring shall not be used in connection with it, and there shall be no sudden changes in the direction of a meridional line at any point. In addition, the radius of curvature of the knuckle in a meridional plane shall be not 18 If, for manufacturing reasons, it is uneconomical or impractical to use butt-welded longitudinal or meridional joints for a distance on either side of the juncture as computed using Equations 24 and 25 and the thickness of the plates involved does not exceed the applicable limits for lap joints as set forth in Table 5-2, the joints may be lap-welded provided that the plates joined in this way are not given credit for contributing to the net cross-sectional area provided for resisting compressive forces in the compression-ring region. In such a case, however, computation of (a) force Q from Equation 26, (b) the width of the horizontal projection (see ) and (c) the centroid of the composite corner compression region (see ) shall be made as though these plates did actually participate in resisting the compressive force. less than 6%, and preferably not less than 12%, of the diameter of the sidewalls. Subject to the provisions of , the thickness of the knuckle at all points shall satisfy the requirements of Use of a knuckle radius as small as 6% of the sidewall diameter will frequently require an excessively heavy thickness for the knuckle region. The thickness requirement for such a region will be found more reasonable if a larger knuckle radius is used The designer should recognize that applying the equations in to levels immediately above and below a point where two surfaces of differing meridional curvature have a common meridional tangent (for example, at the juncture between the knuckle region and the spherically dished portion of a tori spherical roof) will result in the calculation of two latitudinal unit forces, differing in magnitude and perhaps in sign, at the same point. The exact latitudinal unit force at this point will be intermediate between the two calculated values, depending on the geometry of the tank wall in that area; the designer may adjust the immediately adjacent thicknesses accordingly Compression Rings The variables used in Equations are defined as follows: w h = width, in in., of the roof or bottom plate considered to participate in resisting the circumferential force acting on the compression-ring region, w c = corresponding width, in in., of the participating sidewall plate, t h = thickness, in in., of the roof or bottom plate at and near the juncture of the roof or bottom and sidewalls, including corrosion allowance, t c = corresponding thickness, in in., of the cylindrical sidewalls at and near the juncture of the roof bottom and sidewalls, R 2 = length, in in., of the normal to the roof or bottom at the juncture between the roof or bottom and the sidewalls, measured from the roof or bottom to the tank's vertical axis of revolution, R c = horizontal radius, in in., of the cylindrical sidewall at its juncture with the roof or bottom of the tank, T 1 = meridional unit force (see 5.10) in the roof or bottom of the tank at its juncture with the sidewall, in lbf/in. of circumferential arc, T 2 = corresponding latitudinal unit force (see 5.10) in the roof or bottom, in lbf/in. of meridian arc, T 2s = circumferential unit force (see 5.10) in the cylindrical sidewall of the tank at its juncture with the roof

47 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 5-21 or bottom, in lbf/in. measured along an element of the cylinder, α = angle between the direction of T 1 and a vertical line. (In a conical surface it is also one-half the total vertex angle of the cone.) Q = total circumferential force, in lb, acting on a vertical cross section through the compression-ring region, A c = net area, in in. 2, of the vertical cross section of metal required in the compression-ring region, exclusive of all corrosion allowances, S ts = maximum allowable stress value for simple tension, in lbf/in. 2, as given in Table 5-1, E = efficiency, expressed as a decimal, of meridional joints in the compression-ring region in the event that Q should have a positive value, indicating tension (see Table 5-2) If a curved knuckle is not provided, the circumferential compressive forces mentioned in must be resisted by other means in the compression-ring region of the tank walls. This region shall be understood to be the zone of the tank walls at the juncture between the roof or bottom and the sidewalls, including the width of plate on each side of the juncture that is considered to participate in resisting these forces (see Figure 5-5). In no event shall the thickness of the wall plate on either side of the juncture be less than the thickness needed to satisfy the requirements of The widths of plate making up the compression-ring region shall be computed using the following equations: w h = 0.6 R 2 ( t h c) (24) w c = 0.6 R c ( t c c) (25) The magnitude of the total circumferential force acting on any vertical cross section through the compressionring region shall be computed as follows: The net cross-sectional area provided in the compressionring region shall be not less than that found to be required by one of the following equations: The selection of Equation 27 depends on whether the value of Q as determined by Equation is negative or positive Details of Compression-Ring Regions If the force Q is negative, indicating compression, then the horizontal projection of the effective compressionring region shall have a width in a radial direction not less than times the horizontal radius of the tank wall at the level of the juncture between the roof or bottom and the sidewalls; if the projected width does not meet this requirement, appropriate corrective measures shall be applied as specified in this section Whenever the magnitude of the circumferential force Q determined in accordance with is such that the area required by Equation 27 is not provided in a compression-ring region with plates of the minimum thicknesses established by the requirements of 5.10 or when Q is compressive and the horizontal projection of the width, wh, is less than specified in , the compression-ring region shall be reinforced by (a) thickening the roof or bottom and sidewall plates as required to provide a compression-ring region having the necessary cross-sectional area and width as determined on the basis of the thicker plates, 20 (b) adding an angle, a rectangular bar, or a horizontally disposed ring girder at the juncture of the roof or bottom and sidewalls plates, or (c) using a combination of these alternatives. This additional area shall be arranged so that the centroid of the cross-sectional area of the composite corner compression region lies ideally in the horizontal plane of the corner formed by the two members. In no case shall the centroid be off the plane by more than 1.5 times the average thickness of the two members intersecting at the corner Such an angle, bar, or ring girder, if used, may be located either inside or outside the tank (see Figure 5-6) and shall have a cross section with dimensions that satisfy the following conditions: a. The cross-sectional area makes up the deficiency between the area Ac required by Equation 27 and the cross-sectional area provided by the compression-ring region in the walls of the tank. b. The horizontal width of the angle, bar, or ring girder is not less than times the horizontal radius, R c, of the tank wall at the level of the juncture of the roof or bottom and the Q = T 2 w h + T 2s w c T 1 R c sinα (26) 19 Because of the discontinuities and other conditions found in a compression-ring-region, biaxial-stress design criteria are not considered applicable for a compressive force determined as in Equation 26. Experience has shown that a compressive stress of the order of 15,000 lbf/in. 2, as indicated in Equation 27, is permissible in this case, provided the requirements of are satisfied. 20 Note that unless the effect of the unit forces T 2 and T 2s on the A c = Q/15,000 or Q/S ts E (27) resulting increments in width of participating plate may be safely neglected, the use of thicker plats involves recomputing not only T h and W c, but also Q and A for the increased plate thickness; hence the design of the compression-ring-region in this case becomes a trialand-error procedure.

48 5-22 API STANDARD 620 exception, all other part thicknesses and weld sizes referred to in this paragraph relate to dimensions after the deduction of corrosion allowance. Figure 5-5 Compression-Ring Region sidewalls except that when the cross-sectional area to be added in an angle or bar is not more than one-half the total area required by Equation 27, the foregoing width requirement for this member may be disregarded if the horizontal projection of the width, wh, of the participating roof or bottom plates alone is equal to or greater than 0.015R c or, with an angle or bar located on the outside of a tank, the sum of the projection of the width, w h, and the horizontal width of the added angle or bar is equal to or greater than 0.015R c. c. When bracing must be provided as specified in , the moment of inertia of the cross section around a horizontal axis shall be not less than that required by Equation When the vertical leg of an angle ring or a vertical flange of a ring girder is located on the sidewall of the tank, it may be built into the sidewall if its thickness is not less than that of the adjourning wall plates. If this construction is not used, the leg, edge, or flange of the compression ring next to the tank shall make good contact with the wall of the tank around the entire circumference and shall be attached thereto along both the top and bottom edges by continuous fillet welds except as provided in These welds shall be sufficiently sized to transmit to the compression-ring angle, bar, or girder that portion of the total circumferential force, Q, which must be carried thereby, assuming in the case of welds separated by the width of a leg or flange of a structural member as shown in Figure 5-6, details a and h, that only the weld nearest the roof or bottom is effective. In no event, however, shall the size of any weld along either edge of a compression ring be less than the thickness of the thinner of the two parts joined or 1 /4 in. (whichever is smaller), nor shall the size of the corner welds between the shell and a girder bar, such as shown in Figure 5-6, details d and e, be less than the applicable weld sizes in Table 5-8. The part thicknesses and weld sizes in Table 5-8 relate to dimensions in the as-welded condition before the deduction of corrosion allowances; with this If a continuous weld is not needed for strength or as a seal against corrosive elements, attachment welds along the lower edge of a compression ring on the outside of a tank may be intermittent if (a) the summation of their lengths is not less than one-half the circumference of the tank, (b) the unattached width of tank wall between the ends of welds does not exceed eight times the tank wall thickness exclusive of corrosion allowance, and (c) the welds are sized as needed for strength (if this is a factor), but in no case are they smaller than specified in Table The projecting part of a compression ring shall be placed as close as possible to the juncture between the roof or bottom plates and the sidewall plates If a compression ring on either the inside or outside of a tank is shaped in such a way that liquid may be trapped, it shall be provided with adequate drain holes uniformly distributed along its length. Similarly, if a compression ring on the inside of a tank is shaped in such a way that gas would be trapped on the underside when the tank is being filled with liquid, adequate vent holes shall be provided along its length. Where feasible, such drain or vent holes shall be not less than 3 /4 in. in diameter The projecting part of a compression ring without an outer vertical flange need not be braced if the width of the projecting part in a radical vertical plane does not exceed 16 times its thickness. With this exception, the horizontal or near-horizontal part of the compression ring shall be braced at intervals around the circumference of the tank with brackets or other suitable members securely attached to both the ring and the tank wall to prevent that part of the ring from buckling laterally (vertically) out of its own plane. When bracing is required, the moment of inertia of the cross section of the angle, bar, or ring girder about a horizontal axis shall be not less than that computed by the following equation: where Q I p R c l ( ) Q 2 = = pr c 29, 000, 000k k (28) I 1 = required moment of inertia, in in. to the fourth power, for the cross section of a steel 21 compression ring with respect to a horizontal axis through the centroid of the section (not taking credit for any portion of the tank wall) except that in the case of an angle ring whose vertical leg is attached to or 21 The value for I 1 as computed using Equation 28 is not applicable for materials other than steel.

49 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 5-23 Figure 5-6 Permissible and Nonpermissible Details of Construction for a Compression-Ring Juncture

50 5-24 API STANDARD 620 Table 5-8 Minimum Size of Fillet Weld Thickness of the Thicker of the Two Parts Joined (in.) 1 /4 > 1 /4 3 /4 > 3 /4 1 1 /4 > 1 1 /4 forms a part of the tank wall, the moment of inertia of the horizontal leg only shall be considered and shall be figured with respect to a horizontal axis through the centroid of the leg, Q p = that portion of the total circumferential force Q (see Equation 26) that is carried by the compression-ring angle, bar, or girder as computed from the ratio of the cross-sectional area of the compression ring to the total area of the compression zone, R c = horizontal radius, in in., of the cylindrical sidewall of the tank at its juncture with the roof or bottom, k = constant whose value depends on the magnitude of the angle θ subtended at the central axis of the tank by the space between adjacent brackets or other supports, the value of which shall be determined from Table 5-9 in which n is the number of brackets or other supports evenly spaced around the circumference of the tank. In no case shall θ be larger than 90 degrees DESIGN OF INTERNAL AND EXTERNAL STRUCTURAL MEMBERS General The provisions of through are limited to a discussion of the basic requirements and principles involved. For reasons that will appear obvious, specific design formulas cannot be included Basic Requirements Minimum Size of Fillet Weld (in.) 3 / Wherever the shape selected for a tank is such that the tank, or some portion thereof, would tend to assume an appreciably different shape under certain conditions of loading or whenever the shape is such that it is not feasible or economical to design the walls themselves to carry the entire loads imposed by all possible combinations of gas and liquid loadings that may be encountered in service, suitable internal ties, columns, trusses, or other structural members shall be provided in the tank to preserve its shape and to carry the forces that are not carried directly by the walls of the tank. 1 /4 5 /16 3 /8 Other structural members may be needed on the outside of a tank to support or partly support the weight of the tank and its contents, and these shall be provided as required. All such internal and external members shall be designed in accordance with good structural engineering practices, using stresses as specified in 5.6. They shall be arranged and distributed in or on the tank and connected to the walls of the tank (in cases where such connections are needed) in such a way that reactions will not cause excessive localized or secondary stresses in the walls of the tank. When these members are rigidly attached to the wall of a tank by welding, the stresses in the member at the point of attachment shall be limited to the stress value permitted in the wall of the tank (see Appendix D) In no event shall the nominal thickness, including the corrosion allowance, if any, of any part of any internal framing be less than 0.17 in If any structural members (such as girders at node circles), tank accessories, or other internals are placed to form gas pockets inside a tank, adequate and suitably located vent holes shall be provided so that these spaces will vent freely when the liquid level is raised beyond them. Similarly, if any such members, accessories, or other internals are shaped to hold liquid above them when the tank is being emptied, they shall be provided with adequate and suitably located drain holes. These vent and drain holes shall be not smaller than 3 /4 in. in diameter and shall be distributed along the member Simple Systems In some cases the forces acting on structural members are statically determinate; in other cases, they are statically indeterminate. The external columns that are often used for supporting a spherical tank are an example of the statically determinate class of members. If the columns are vertical, the force acting on each column is simply the combined weight of the tank and its contents divided by the number of columns. If the columns are inclined, this quotient must be divided by the cosine of the angle each column makes with the vertical to obtain the force acting in each column. To cite another case, where internal framing is needed inside a tank only to support the weight of the roof and such loads (including external pressure load, if any) as may be superimposed upon it, the procedure for designing such framing is more or less straightforward, involving only a few assumptions. In other cases, however, whenever the internal framing serves to supplement the load-carrying capacity of the walls of the tank, the design procedure is more complex Complex Systems The design rules in this standard do not cover specific requirements for designing the internal framing in all the various shapes of tanks that might be constructed, but an out-

51 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 5-25 line of the procedure used in the design of internal framing for one special shape of tank, as shown in Figure 5-4, panel c, should serve to illustrate the general method of attack. In such a system of internal framing, the magnitude of the forces in the tension members, which tie the ring girders under the roof node circles to the respective girders above the bottom node circles, are determined by static, assuming for the purpose of a preliminary analysis that these tension members are replaced by a cylindrical shell if the members are vertical or by a conical frustum if the members are inclined Under these assumed conditions, the vertical components of the T 1 (meridional) unit forces in the roof plates at their juncture with the cylinder or frustum are transmitted directly to the cylinder or frustum so that an upper ring girder is unnecessary in this hypothetical case if (a) the horizontal components of the T 1 unit forces in the roof or wall plates on opposite sides of the juncture balance each other in the case of the cylindrical tie or (b) the difference between them is balanced by the horizontal components of the unit forces in the top of the frustum in the case of the conical tie Similarly, at the lower end of the cylinder or frustum, the summation of the vertical components of the forces must be in balance with the vertical components of the forces in the cylinder or frustum, and the summation of the horizontal components of the forces acting at the juncture must be zero. Furthermore, the total vertical force acting along the edges of the top of the cylinder or frustum must equal the total vertical force acting along the edges of the bottom of the cylinder or frustum. In other words, the general layout of the tank must be such that the upward gas pressure over a predetermined portion of the roof is balanced by the downward gas pressure over a predetermined portion of the bottom without undue elastic stressing or straining If the horizontal forces at the node circles are not otherwise in equilibrium, ring girders must be provided at these circles. The girders must be designed to carry the unbalanced components either in tension or compression, as the case may be Having satisfied the conditions of static equilibrium using a hypothetical cylinder or frustum for a tie, the designer must consider and provide for the real conditions where the cylinder or frustum is approximated by a number of uniformly spaced structural members, each of which, in addition to its primary function as a tie, serves also as a column to support its assigned portion of the roof and external loads. The torsional and vertical moments in the ring girders at the node circles must be provided for, keeping in mind that relatively small variations from the nominal T 1 (meridional) roof forces will greatly reduce, if not completely offset, the torsional moments in the girders. Table 5-9 Factors for Determining Values of k for Compression-Ring Bracing (See ) n θ (degrees) Internal Meridional Stiffeners When curved meridional trusses or ribs are fastened to the sidewalls of a tank to prevent the T 1 (meridional) compressive forces from buckling the sidewalls, the distribution of meridional forces between the sidewalls and trusses or ribs is to a degree indeterminate if the foundation support for the overhanging portions of the sidewalls is so uniformly distributed around the tank that there is no greater foundationbearing intensity against the tank wall beneath the trusses or ribs. In this case, the total meridional forces that the sidewalls and trusses or ribs must carry, acting together, at any given level in the tank may be computed from Equation 1 in , assuming for the purposes of these computations only that the cross-sectional area of the trusses or ribs is distributed uniformly along the circumference of the sidewalls as an added sidewall thickness. In other words, the value of F in Equation 1 may be taken as not including the forces in these trusses or ribs, and the hypothetical value of the meridional unit force computed using Equation 1 may be regarded as the summation of all meridional forces acting on the composite section of sidewalls and trusses or ribs at the level under consideration divided by the circumference of the tank at that level The net cross-sectional area of metal (exclusive of corrosion allowance) required per inch of tank circumference to resist these forces may then be determined by dividing the hypothetical value of the meridional unit forces acting on the composite section by allowable compressive stress. This area must then be apportioned between the sidewalls and the trusses of ribs, by trial-and-error computations, in such a way k

52 5-26 API STANDARD 620 that (a) sufficient material is placed in the trusses or ribs to enable them to serve their intended function of preventing buckling of the sidewalls in a vertical direction (the trusses or ribs must also be proportioned and distributed around the circumference of the tank so that they will serve this function) and (b) sufficient thickness is provided in the sidewalls to enable them to withstand not only their share of the meridional unit forces but also the entire latitudinal unit force T 2 as computed by the following equation: T 2 = R 2 (P T 1 /R 1 ) In this equation, T 1 is the meridional unit force assumed to be actually carried by the sidewalls and is obtained by multiplying the hypothetical value of the meridional unit forces acting on the composite section by the ratio of the sidewall cross-sectional area to the composite cross-sectional area at the level in question. Other variables used in the foregoing equation are defined in , and the thickness provided to resist this force T 2 must satisfy all of the requirements of that involve this force No such uniform distribution of forces on the composite section of sidewalls and trusses or ribs actually occurs. However, the assumption of uniform distribution of and will give safe designs if the principles outlined are observed and the eccentricity of loading on the trusses or ribs is taken into account. (New designs shall be proved by strain-gauge surveys.) In the case of a tank whose foundations and supports are designed and arranged so that the weight of the overhanging portions of the tank and its contents is transferred entirely to the trusses or ribs and from there to the foundations, the total vertical load on each truss or rib is determinate. The stress system in the tank wall is analogous to that in a large horizontal pipeline supported entirely on ring girders. In the latter case, design stresses comparable to those permitted in may be used insofar as sidewall thicknesses are governed by forces acting in a meridional direction SHAPES, LOCATIONS, AND MAXIMUM SIZES OF WALL OPENINGS The term opening as used in this section, 5.16, 5.17, and 5.18 refers to the hole cut in a tank wall to accommodate a nozzle, manway, or other connection (rather than just the bore of the connection) except when the wall of a connection extends through the tank wall and is attached to it with sufficient weld within the tank wall thickness to develop the strength in tension of that section of the wall of the connection which lies within the tank wall thickness (that is, the strength of an area equal to twice the product of the nozzle wall thickness and the tank wall thickness) in addition to whatever welding is required at this location for reinforcement attachment. In the latter case, when the wall of a connection is attached to the tank wall in this way, opening refers to the figure formed by the imaginary line of intersection between the inside surface of the connection and the surface of the tank wall extended In all cases, requirements concerning openings shall be understood to refer to dimensions that apply to the corroded condition. Unless otherwise specified, dimensions of openings generally refer to measurements taken along the chord of the tank wall curvature if the wall is curved in the direction involved; however, when there is more than approximately a 2% difference between the length of chord and the length of the arc that is subtends in the tank wall, the measurement shall be taken along the arc of the tank wall curvature The rules in this section shall also apply to openings in cylindrical shells that are adjacent to a relatively flat bottom; as an alternative, the insert plate or reinforcing plate may extend to and intersect the bottom-to-shell joint at approximately 90. Stress-relieving requirements do not apply to the weld to the bottom or annular plate All manholes, nozzle connections, or other connections in the sidewalls, roofs, or bottoms of tanks constructed under these rules shall be circular, elliptical, 22 or obround 23 in shape. Where elliptical or obround connections are employed, the long dimensions shall not exceed twice the short dimension, as measured along the outer surface of the tank; if the connection is in an area of unequal meridional and latitudinal stresses in the tank wall, the long dimension shall preferably coincide with the direction of the greater stress Each opening in the walls of a tank shall be located so that the distance between the outer edge of its reinforcement 24 and any line of significant discontinuity in the curvature of the tank walls (such as the juncture between two nodes in a noded surface, the juncture between a dished or conical roof or bottom and cylindrical sidewalls, or the juncture between a roof or bottom and cylindrical sidewalls, or the juncture between a roof or bottom knuckle and other portions of the tank) is not less than 6 in. or (if this be larger) eight times the nominal thickness (including corrosion allowance; if any) of the wall plate containing the opening, except as permitted by No part of the attachment for any openings shall be located closer than the larger of these distances to any 22 An opening made for a pipe or nozzle of circular cross section whose axis is not perpendicular to the tank wall shall be treated as an elliptical opening for design purposes. 23 An obround figure is one that is formed by two parallel sides and semi-circular ends. 24 The term edge of reinforcement means the edge, or toe, of the outermost weld that attaches the reinforcing pad to the wall of the tank. In the case of an opening that is not provided with a reinforcing pad, it means the neck of the nozzle or other connection extending from the opening to the tank wall.

53 DESIGN AND CONSTRUCTION OF LARGE WELDED, LOW-PRESSURE STORAGE TANKS 5-27 part of the attachment for any lugs, columns, skirts, or other members attached to the tank for supporting the tank itself or for supporting important loads that are carried by the tank. When any two adjacent openings are reinforced independently of each other, they shall be spaced so that the distance between the edges of their respective reinforcements will not at any point be less than the larger of the foregoing specified distances (see 5.17) Each opening shall be located so that any attachments and reinforcements will be, or may readily be made, fully accessible for inspection and repair on both the outside and inside of the tank except in the case of connections that for compelling reasons must be located on the underside of a tank bottom resting directly on the tank foundation Properly reinforced openings may be of any size 25 that can be located on the tank to comply with the requirements of and except that in no event shall the inside diameter (after allowing for corrosion) of any opening 26 other than those considered in 5.18 exceed 1.5 times the least radius of curvature in that portion of the tank wall in which the opening is located Large openings shall be given special consideration (see and 5.18). In the case of large openings which have attachments that require shop stress relief (see ), shipping clearances, affecting the maximum size of assembly that can be shipped, may control the size of the opening that can be used INSPECTION OPENINGS Each tank shall be provided with at least two manhole openings to afford access to its interior for inspection and repair. Manholes shall in no event be smaller than 20 in. along any inside dimension. All manholes shall be made readily accessible by platforms and ladders, stairways, or other suitable facilities REINFORCEMENT OF SINGLE OPENINGS General The requirements of this paragraph are illustrated in Figures 5-7 and 5-8. See , , , and for provisions concerning reinforcement of openings in cover plates for nozzles Basic Requirements All openings in the walls of tanks constructed according to these rules and all openings for branch connections 27 from nozzle necks welded to the tank wall shall be fully reinforced with the exception of the exclusions covered in and Single openings in tanks do not require reinforcement other than that which is inherent in their construction for the following conditions: a. Three in., or less, pipe size welded connections in tank walls 3 /8 in. or less. b. Two in., or less, pipe size welded connections in tank walls over 3 /8 in. c. Threaded connections in which the hole in the tank wall is not greater than 2 in. pipe size The reinforcement required for openings in tank walls for external pressure conditions need be only 50% of that required in where t has been determined for external pressure conditions The requirements for full reinforcement shall not be construed as requiring that a special reinforcing pad be provided where the necessary reinforcing metal is available in the nozzle neck or elsewhere around the opening as permitted by these rules. The amount of reinforcement required, the limiting dimensions within which metal may be considered to be effective as reinforcement, and the strength of the welding required for attaching the reinforcement are defined in Reinforcement shall be provided in the specified amount and shall be distributed and attached to the wall of the tank in such a way that the requirements are satisfied for all paths of potential failures through the opening extended in either a meridional or latitudinal direction The maximum amount of reinforcement will be needed in a plane that is perpendicular to the direction of principal wall stress passed through the opening at the point where the centerline of the connection intersects the wall of the tank; for obround openings, that same amount must be provided along the entire length of the parallel sides of the opening between the planes passing through the respective centers of the semicircular ends. However, these planes may not be the controlling sections with respect to possible failure through the opening, inasmuch as failure might occur along another path (in the case of a cylindrical wall, parallel to, but somewhat removed from, the aforesaid planes) by a combina- 25 Although no minimum size is prescribed, it is recommended that no nozzle smaller than 3 /4 in. pipe be used on a tank constructed according to these rules. 26 In the case of elliptical or obround openings, the dimension of the opening in any given direction shall meet this requirement with respect to the radius of curvature of the tank wall in that direction. 27 The design rules in this section make no mention of openings for branch connections from nozzle necks, but the provisions shall be understood to apply to openings of this type. For this purpose, the term tank wall shall refer to the neck of the main nozzle to which the branch connection is attached, and the term nozzle wall shall refer to the wall of the branch connection.

54 5-28 API STANDARD (t w - c) 2.5(t w - c) Use the smaller dimension G (d + 2c) 2 + (tn - c) + (t w - c) t r = Required thickness of a seamless wall (or solid plate); if the opening passes through a welded joint whose direction is parallel to the cross section under consideration, t r shall be given a value that allows for the difference, if any, between the specified efficiency of the joint and an efficiency of 100%. t m = Required thickness of the nozzle neck, allowing for the efficiency of longitudinal joints, if any, in the nozzle neck. e 1 = Excess thickness if the opening is in solid plate. e 2 = Excess thickness if the opening passes through a welded joint that has an efficiency of less than 100% and whose direction is parallel to the cross section under consideration. e 3 = Thickness of the nozzle neck available for reinforcement of the opening. A 1 = Area in excess thickness of the tank wall that is available for reinforcement. A 2 = Area in excess thickness of the nozzle neck that is available for reinforcement. A 3 = Cross-sectional area of welds available for reinforcement. A 4 = Cross-sectional area of material added as reinforcement. tion of a tensile failure of the tank wall and shearing or tensile failure of the attachment welds Size and Shape of Area of Reinforcement The area of reinforcement for a given cross section of an opening shall be understood to be that area in a plane normal to the surface of the tank and passing through the section under consideration within which available metal may be deemed effective for reinforcing the opening. For surfaces that have straight elements, such as cylinders and cones, the areas of reinforcement will be rectangular in shape as indicated by lines GH, HK, GJ, and JK in Figure 5-7; however, on surfaces that are curved in two directions, the lines GH and JK shall follow the contour of the tank surface The maximum length of the area of reinforcement shall be the greater of the following limiting distances on each side of the axis of the opening, measured along the outside surface of the tank: or 2.5(t n - c) J (d + 2c) Note: See and Figure 5-8 for defintions of other variables. t n d d + 2c t r e 1 e 2 A4 t n - c t m e 3 A 2 A 3 A 4 Use the greater dimension K Figure 5 7 Reinforcement of Single Openings c A 3 a. A distance equal to the diameter of the opening after corrosion; in the case of non circular openings, a distance equal to the corresponding clear dimension is substituted for the diameter of the opening. b. A distance equal to the radius of the opening after corrosion plus the thickness of the nozzle wall plus the thickness of the tank wall, all taken in the corroded condition; in the case of non circular openings, a distance equal to the corresponding half chord is substituted for the radius of the opening. H A 1 (t - c) c t t w The maximum width of the area of reinforcement, measured radially as applicable from either the inner or outer surface of the tank wall, or both, shall be not more than the smaller of the two following distances: a. A distance equal to 2.5 times the nominal thickness of the tank wall less the corrosion allowance. b. A distance equal to 2.5 times the nominal thickness of the nozzle wall less its corrosion allowance plus the thickness of any additional reinforcement inside or outside the tank wall